docs: add session projection design skill

AGENTS.md

@@ -135,6 +135,7 @@ Real-API tests/demos read `DEEPSEEK_API_KEY`, optional `DEEPSEEK_BASE_URL`, and
 - **Switch on discriminant tags.** Closed unions end in `assertNever`; merge-extensible unions fall through a documented default.
 - **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.
+- **Session projections:** follow [dsh-session-projection](.agents/skills/dsh-session-projection/SKILL.md) when creating, extending, reviewing, or migrating one.
 - **Plugins, not loop changes**: new behavior goes on documented extension points; changing `agent-loop` requires updating docs/architecture.md.
 - **A capability seam comprises Service Definition / Service Provider / Consumer roles.** It is complete, never one role; split only when roles evolve independently ([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)).