feat: project-scope chat threads aggregate descendant FILING notes (v1.20.0)

[PaulEastabrook]

Stage 2b — the deferred cross-child increment of board-chat enrichment § 2.8

Stacked on #29 (v1.19.0) — merge that first; GitHub retargets this to main automatically.

What

When gtd_chat_thread's target task carries #project, the files[] FILING scan additionally covers the project's descendant tasks (children + grandchildren — the same ≤3-level tree gtd_project_plan walks; completed included, since a completed action's filed output is still a project output). Each descendant-filed entry carries additive provenance fields item_id/item_name (the descendant that filed it).

Triggering case: the project-scope EM thread (rtm:1179319604) — "what outputs have we produced?" correctly returned files: [] on v1.19.0 because the four filed packs' OUTPUT notes live on the child actions, not the project task.

Unchanged (by design)

• Same one-call read — the broad getList already carries the children; call surface stays exactly ["rtm.tasks.getList"] (asserted in-suite).
• Same OUTPUT-title-gated parser, same conservative correlation, paths byte-verbatim (dedup guarantee holds).
• Item-scope threads byte-identical to v1.19.0 — no provenance fields, same-task only (regression-guarded). The gate is the #project tag, not subtask presence.
• No board change needed; no new tag, no strict-tag interaction, no activation-ordering hazard.

Verification

• make test: 736 passed (+14: 12 pure, 2 FakeMCP tool tests — child/grandchild/completed-child provenance, after-last-ai-turn unattached, two-window discipline across children, item-scope shape regression, non-#project-with-subtask same-task-only, read-only call surface).
• make lint clean (ruff check + format + pyright).
• Not run: live smoke (needs server restart on v1.20.0 + Cowork-side OUTPUT-note backfill) — the EM thread is the designated acceptance test.

Docs in lockstep (§ 9): README, server.py instructions, CLAUDE.md feature section + test inventory. MINOR → v1.20.0 (pyproject + __init__ + uv.lock). Debrief: project-scope-chat-filings-debrief.md (§ 14).

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