feat(subagent): opportunistic projection-cache rung for cold listings

Cold children consult the optional session-projection-cache checkpoint
before paying a preparation recompute: the identity is immutable once
appended, so a cached value is definitive regardless of its watermark.
The cache stays a read-only accelerator (absent service or any rung-two
fault falls through silently; verdicts stay with the authoritative
refold), and the note plus core-data-structures pages describe the
three-rung ladder.

docs/core-data-structures/subagent.md

@@ -252,7 +252,7 @@ A local one-shot provider appends the descriptor inside the child's initial turn
 
 ## Durable enumeration: `listChildren()` and `SubagentListEntry`
 
-`SubagentService.listChildren(parentSessionId)` enumerates the parent's direct session-backed subagents from the live-preferred merge of `ctx.sessions.list()` and optional `ctx.sessionPersistence.list()` — no query seam, and no Agent is loaded or resumed. Candidates are the direct children whose durable header carries `origin: 'subagent'`; the marker classifies enumeration and coarse generic-route denial but cannot establish a valid descriptor, resumability, or authorization — the projection fold owns identity, and the Activation contract owns resume. Each row's `mode`/`label` is the registered `subagent` projection unit's value, served from the registry's watermark cache for a live child (zero log reads) and folded once over one `persistence.inspect()` reading for a cold one (bounded concurrency, recomputed per listing — no cache). The fold is `subagent/descriptor` last-wins with no failure channel: the child's own descriptor overrides a fork-seeded ancestor's, and a malformed or unknown-version payload folds to no value. The result is one `SubagentListEntry[]` in `createdAt`-then-id order: a served identity yields a `child` entry with `mode: 'one-shot' | 'continuable'` and `activity: 'running' | 'inactive'`; continuable entries always carry `label`, while one-shot entries carry it only when the start caller supplied presentation metadata. A settled candidate whose fold served no identity yields a `corrupt` diagnostic — missing, malformed, and unknown-version descriptors deliberately undistinguished, with `unsupported` kept in the type for consumers already routing on it but no longer produced; a running candidate without an identity is omitted (the creation window before its descriptor lands); a failed cold inspection yields one `unavailable` diagnostic retried on the next listing, so one damaged sibling cannot hide healthy children. `hasChildren` marks a direct descendant with durable subagent origin, read from the same merged material. Activity snapshots only whether the logical record is live in `ctx.sessions`, not outcome or resumability. Absent persistence, enumeration is live-only rather than an error — a cold child cannot be resumed then either. `listChildren()` throws `SubagentError` with code `SUBAGENT_CONTROL_PROJECTIONS_UNAVAILABLE` when the `ctx.sessionProjections` registry is absent, checked before any read so a deployment with zero children still fails deterministically; the list tool requires `ctx.subagents` at plugin load. A service consumer such as a UI can display both modes and choose an unlabeled one-shot fallback, while the model-facing `list_agents` adapter (the separately loadable `/list-agents` plugin of [dsh-tool-subagent-control](../../packages/subagent/tool-subagent-control)) keeps only continuable entries and maps activity to its existing `running`/`complete` vocabulary. Listing does not consult the continuation manager's Activation map, Agent registry, or provider availability; `send_message` remains the authoritative delivery-time operation, and a listed running continuable child may still reject delivery as an ownership conflict. The read-path rationale lives in [the list-identity-projection Agent Note](../../.agents/notes/implemented/architecture/2026-08-06-subagent-list-identity-projection.md).
+`SubagentService.listChildren(parentSessionId)` enumerates the parent's direct session-backed subagents from the live-preferred merge of `ctx.sessions.list()` and optional `ctx.sessionPersistence.list()` — no query seam, and no Agent is loaded or resumed. Candidates are the direct children whose durable header carries `origin: 'subagent'`; the marker classifies enumeration and coarse generic-route denial but cannot establish a valid descriptor, resumability, or authorization — the projection fold owns identity, and the Activation contract owns resume. Each row's `mode`/`label` is the registered `subagent` projection unit's value, served through a three-rung ladder: the registry's watermark cache for a live child (zero log reads); the optional projection checkpoint cache for a cold one (`cachedSnapshot` — a served identity is final, because identity is immutable once appended); otherwise one `persistence.inspect()` reading folded through the registry (bounded concurrency, recomputed per listing). The cache is a pure optional accelerator: absent, missing the key, or faulting, it falls silently through to the authoritative refold. The fold is `subagent/descriptor` last-wins with no failure channel: the child's own descriptor overrides a fork-seeded ancestor's, and a malformed or unknown-version payload folds to no value. The result is one `SubagentListEntry[]` in `createdAt`-then-id order: a served identity yields a `child` entry with `mode: 'one-shot' | 'continuable'` and `activity: 'running' | 'inactive'`; continuable entries always carry `label`, while one-shot entries carry it only when the start caller supplied presentation metadata. A settled candidate whose fold served no identity yields a `corrupt` diagnostic — missing, malformed, and unknown-version descriptors deliberately undistinguished, with `unsupported` kept in the type for consumers already routing on it but no longer produced; a running candidate without an identity is omitted (the creation window before its descriptor lands); a failed cold inspection yields one `unavailable` diagnostic retried on the next listing, so one damaged sibling cannot hide healthy children. `hasChildren` marks a direct descendant with durable subagent origin, read from the same merged material. Activity snapshots only whether the logical record is live in `ctx.sessions`, not outcome or resumability. Absent persistence, enumeration is live-only rather than an error — a cold child cannot be resumed then either. `listChildren()` throws `SubagentError` with code `SUBAGENT_CONTROL_PROJECTIONS_UNAVAILABLE` when the `ctx.sessionProjections` registry is absent, checked before any read so a deployment with zero children still fails deterministically; the list tool requires `ctx.subagents` at plugin load. A service consumer such as a UI can display both modes and choose an unlabeled one-shot fallback, while the model-facing `list_agents` adapter (the separately loadable `/list-agents` plugin of [dsh-tool-subagent-control](../../packages/subagent/tool-subagent-control)) keeps only continuable entries and maps activity to its existing `running`/`complete` vocabulary. Listing does not consult the continuation manager's Activation map, Agent registry, or provider availability; `send_message` remains the authoritative delivery-time operation, and a listed running continuable child may still reject delivery as an ownership conflict. The read-path rationale lives in [the list-identity-projection Agent Note](../../.agents/notes/implemented/architecture/2026-08-06-subagent-list-identity-projection.md).
 
 ## The terminal result: `SubagentResult`