feat(session-title): add fallback and model providers

packages/session-title/session-title/README.md

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+# @deepseek-ai/dsh-session-title
+
+Log-backed session titles with an immediate deterministic fallback and one optional asynchronous provider. Every accepted revision is a log-only `session/title` event; `foldSessionTitle()` and `ctx.sessionTitle.get()` select the latest event and return its event seq and timestamp.
+
+Only text blocks from human `user/message` events are eligible. The first eligible prompt schedules a fallback from its first words within the configured UTF-8 byte limit. Whitespace is normalized, terminal control sequences are removed, and truncation never splits a code point. Empty and non-text prompts wait for later eligible input.
+
+## Service: `SessionTitleService` (ctx key: `sessionTitle`)
+
+- `get(session)` folds the latest accepted title from a live or replayed log.
+- `refresh(session, signal?)` materializes the fallback when needed, then explicitly runs the registered provider over the current eligible messages. Provider errors and caller cancellation reject.
+- `register(provider)` installs the sole optional provider and returns its Cordis effect disposer. A second registration throws immediately; disposal aborts pending and active calls before another provider can register.
+
+Automatic work never delays the main agent response. A provider starts after the matching `request/header` records the main request's exact route; its late completion joins an open turn or uses a flushed zero-step `session-title` turn through `ctx.sessions.appendOutOfBand()`. Automatic failures warn and retain the latest title. New all-message revisions, provider disposal, session disposal, and explicit refresh abort older work, and a stale completion cannot append.
+
+Forks inherit title events in their seed unchanged. The first-message cadence does not automatically retitle a child; the all-messages cadence may append a new revision after the child receives a later human prompt.
+
+## Configuration
+
+All limits are required; the library supplies no defaults.
+
+| Key | Contract |
+|---|---|
+| `fallbackMaxWords` | Positive maximum whitespace-delimited words in the deterministic fallback. |
+| `fallbackMaxBytes` | Positive maximum UTF-8 bytes in the fallback; must not exceed `maxTitleBytes`. |
+| `maxTitleBytes` | Positive maximum UTF-8 bytes accepted from any source. |
+
+## Provider contract
+
+A provider supplies a branded stable id, automatic mode (`first-message` or `all-user-messages`), and `generate(request)`. The request carries the live session, all eligible messages through one fixed revision, the current logged main-request route when available, and cancellation. The result identifies a non-empty title, unique ordered source-message seqs from that request, and optional model provenance. The service normalizes and validates the result before it becomes durable.
+
+See the [session-title data structures](../../../docs/core-data-structures/session-title.md) and [implemented decision](../../../.agents/notes/implemented/feature/2026-07-21-log-backed-session-titles.md).
+
+## Model Experience
+
+### Session title state
+
+#### What the model sees
+
+Nothing. `session/title` is log-only and never enters the session surface, `deriveMessages()`, system prompt, tool schemas, or request prefix.
+
+#### Token effect
+
+The fallback and accepted provider revisions add zero tokens to the main agent request. An optional provider's separate auxiliary request is documented by that provider package.
+
+#### KV Cache effect
+
+None for the main request; title events do not change its reconstructed content or cache key.
+
+## Known Limitations and Deferred Work
+
+- Manual rename, title deletion, generated-versus-user precedence, search, and list indexing are outside this service.
+- The provider registry deliberately accepts at most one implementation, so a deployment cannot compose competing title strategies without writing one provider that owns their precedence.