docs: define a seam as the (Service, Service provider, Consumer) trio

AGENTS.md

@@ -104,7 +104,7 @@ Real-API tests and demos read `DEEPSEEK_API_KEY`, optional `DEEPSEEK_BASE_URL`,
 - **Waterfall listeners MUST call `next()`** to delegate; returning without it short-circuits the chain ([semantics](docs/cordis-primer.md#cordis-waterfall-semantics)).
 - **Model-visible ⟺ logged**: anything that reaches a model request must be reconstructable from the session log; a new model-visible input requires a session event.
 - **Plugins, not loop changes**: new behavior goes on the documented extension seams; changing `agent-loop` requires updating docs/architecture.md.
-- **Capability seams are three packages** — interface / implementation / consumer; don't split preemptively.
+- **Capability seams span three packages** — interface / implementation / consumer; don't split preemptively. The seam is all three ([glossary](docs/glossary.md#capability-seam)).
 - **Prefer maintained dependencies over hand-rolling** when they genuinely delete owned code and tests ([policy](.agents/notes/implemented/process/2026-07-26-dependencies-over-hand-rolling.md)).
 - **Explicit > implicit at package seams**: defaulting is an explicit `resolve(request): Spec` step in the owning implementation, never a hidden `?? default` inside `run()` (the `dsh-bash` request/spec split is the template).
 - **No hardcoded tunables in plugins**: deployment-varying choices are validated `Config` fields changeable from cordis.yml; a `DEFAULT_*` constant or test seam is not configurability. Protocol constants, external specs, and security invariants stay fixed.