docs: restructure docs into diataxis (#121)

docs/reference/agents/invariants.md

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+# Invariants
+
+These are the “don’t break this” rules that keep Takopi reliable.
+
+## Runner contract
+
+The runner contract is enforced by `tests/test_runner_contract.py`:
+
+- Exactly one `StartedEvent`
+- Exactly one `CompletedEvent`
+- `CompletedEvent` is last
+- `CompletedEvent.resume == StartedEvent.resume`
+
+See also the [Plugin API](../plugin-api.md) runner contract section.
+
+## Per-thread serialization
+
+At most one active run may operate on the same thread/session at a time.
+This is enforced both by scheduling and by per-resume-token runner locks.
+
+Normative details live in the [Specification](../specification.md) (§5.2).
+
+## Resume lines
+
+Resume lines embedded in chat are the engine’s canonical resume command (e.g. `claude --resume <id>`).
+
+- The runner is authoritative for formatting and extraction.
+- Transports/rendering must preserve the resume line reliably (even when trimming/splitting).
+
+Normative details live in the [Specification](../specification.md) (§3).
+
+## Local contribution hygiene
+
+- Run `just check` before code commits.
+