docs(compact): align seam docs with the pre-step seam and record session/invariants changes (CBR-003)

Codex round 1 CBR-003: several docs still described compaction as an
`agent/request` waterfall concern, and the implemented compaction RFC
claimed "No changes to dsh-session or dsh-invariants" while the diff
changed both.

- Package READMEs / JSDoc (agent, agent-loop, system-prompt, compact,
  compact-basic): compaction now lives on the serial `agent/pre-step`
  seam (fired after turn/start, before step/start); the structural guard
  is tool-pairing balance (`isToolPairingBalanced`), not step-alignment;
  the convergence bound is strict (`>=` rejects).
- architecture.md / core-data-structures/compaction.md: same seam +
  predicate + dispatch-mode updates; regenerated cordis catalog.
- Implemented compaction RFC, updated in place to describe shipped
  reality: the seam is `agent/pre-step` (@mode serial) fired before
  step/start; alignment is surface tool-pairing balance; the convergence
  invariant rejects `>=`; and the "no dsh-session/dsh-invariants changes"
  claim is corrected — dsh-session gains the tool-pairing predicate and
  dsh-invariants drops its `start <= end` replace assertion (a positional
  replace makes start > end normal).

packages/core/agent-loop/README.md

@@ -52,6 +52,8 @@ forever:
     STEP loop:
       drain steering
       assembly = systemPrompt.assemble()
+      await serial agent/pre-step        ⟵ surface mutation (compaction) outside the step
+      session('step/start')
       request = waterfall agent/request
       stream llm.stream(request) → session('assistant/chunk')
       message = waterfall agent/step-result
@@ -73,8 +75,8 @@ Cancellation: `agent.cancel()` is the single public stop primitive — it clears
 ### What is NOT here
 
 Everything that goes beyond "call the model, run the tools, repeat" belongs to plugins listening on the event taxonomy:
-- Hooks: `agent/request`, `agent/step-result`, `tools/execute`, `agent/turn-continuation`
-- Compaction: `agent/request`
+- Hooks: `agent/pre-step`, `agent/request`, `agent/step-result`, `tools/execute`, `agent/turn-continuation`
+- Compaction: `agent/pre-step`
 - Sandbox, permission, plan mode: `tools/execute`
 - Sub-agents: TODO seam on `AgentLoop.create()`
 - Persistence: `session/event` + `session/flush`