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/system-prompt/README.md

@@ -34,4 +34,4 @@ Merge-extensible: plugins can declare extra fields on `PromptAssembly` via decla
 ### What is NOT here
 
 - Any hardcoded prompt text — every section comes from plugins.
-- Prompt compaction (belongs on the `agent/request` seam in `dsh-agent`).
+- Prompt compaction (belongs on the `agent/pre-step` seam in `dsh-agent`).