feat(extraction): drop candidates that echo a recent manual store

[gorkem2020]

Problem

When a user stores a fact manually with memory_store (or memory_update), the same conversation turn usually still flows through auto-capture extraction. The extractor then re-emits the just-stored fact as a candidate, and the pipeline spends judge, dedup, and merge LLM calls deciding what to do with a row that already exists verbatim. In the noisiest case the near-duplicate survives as a second row.

Fix

A small deterministic echo guard:

• Manual memory_store / memory_update texts are recorded in an in-memory per-agent ring (8 entries, no persistence, no config surface).
• Extraction candidates near-identical to a recorded text are dropped before the admission judge, with an INFO log line. Matching is deterministic and cheap: normalized containment, token-subset ratio >= 0.9 (both directions), or Jaccard >= 0.75. No LLM calls, no vector reads.
• The manual row itself is untouched; it was already stored verbatim by the manual-priority lane.

Paraphrase-level echoes (same meaning, different words) are deliberately out of scope: covering them would need embedding lookups and would risk dropping genuinely new memories. The word-level tiers plus the existing dedup layer cover the observed failure shape.

Notes

• Stacked on #960 (manual-priority supersede) because both touch the same tools.ts region; this diff is the last commit on the branch. Best reviewed after feat(tools): manual-priority supersede for memory_store #960 merges.
• New unit suite test/manual-echo-guard.test.mjs (17 cases: tier matches, short-text guard, per-agent isolation, ring eviction); registered in the test chain and CI manifest.

🤖 Generated with Claude Code

[gorkem2020]

Recomposed against current master (post-#959/#947 merges) as a single commit carrying only this family's delta. Full suite green locally. Ready for review.