feat(context): slice 1 — non-destructive working set with age-horizon policy and exact recall

[akougkas]

Part of #179.

Entry kinds contextEviction / contextRecall (format v4, forward rejection in runMigrations), fold.ts, project.ts, byte-stable markers, policies/age-horizon.ts reproducing today's selection through the ledger, engine.ts replacing the mask stage in runAutoCompact (no replaceEntries), context(scope="recall"), /context working-set section, transcript evicted tag, context.workingSet settings block (enabled: true, policy: age-horizon). Legacy destructive mask behind CLIO_CODER_LEGACY_MASK=1 for one release.

Acceptance

• Scripted runAutoCompact appends one contextEviction and never calls replaceEntries; with enabled: false the summary stage is reached directly.
• age-horizon selects the same set of entries maskStaleObservations would have rewritten (floor disabled).
• Recall round-trips byte-exact and appends contextRecall; off-path and not-evicted refs return typed errors naming the nearest valid ref.
• Fork and /tree cases pass through filterEntriesToActivePath (fix(session): /tree switch and /fork reconstruct different state from the same turn #94).
• Every switch (entry.kind) site handles the new kinds (exhaustiveness test).
• npm run ci green.

[akougkas]

Landed on v0.3.4 via PR #183, range 8a1c8304..9073e395 (merge f404745f).

Against the acceptance list:

• Scripted runAutoCompact appends one contextEviction and never calls replaceEntries; enabled: false reaches the summary stage directly (tests/contracts/context-working-set-wiring.test.ts).
• age-horizon selects the set maskStaleObservations would have rewritten with the floor disabled (working-set-age-horizon.test.ts); with the default floor, results under minEvictableTokens now stay, which the docs state.
• Recall round-trips byte-exact and appends contextRecall. The "nearest valid ref" part of the acceptance was dropped on review: over time-ordered ids the prefix guess named an unrelated result, so every failure lists the recallable refs with their tool and path instead (ba0c311d).
• Fork and /tree go through filterEntriesToActivePath (working-set-fold.test.ts, scenarios S9).
• Every switch (entry.kind) site handles the new kinds (working-set-entry-kinds.test.ts).
• npm run ci at 9073e395: typecheck, lint (check-hygiene 13/13), skills:check, build, 24 test shards with 4529 pass and 0 fail, trace-viewer tests.

Two deviations from the issue text, both deliberate: policy defaults to structural-v1 (decided in #181), and a version 3 session migrates to 4 in place rather than being refused, since the bump is additive.