ADRs + agent-rules sync

[LukeMainwaring]

Stage 3 of 3 in the refactor series (follows #66 and #67). Docs and agent-rules only — no app code changes.

Part A — docs/adr/

Four rationales that already lived in code comments are now numbered records, and each source cites its ADR back (per .claude/rules/conventions.md: "the docstring that implements it cites the ADR").

ADR	Summary
0001-hnsw-over-ivfflat	IVFFlat would be trained on an empty table at migration time and need a REINDEX after every seed pass; HNSW builds on empty and has better recall at our ~2k–10k rows.
0002-itunes-previews-over-spotify-preview-url	Spotify deprecated preview_url for standard-mode apps (0/10 hits here), so iTunes Search supplies audio bytes while Spotify stays the identity source; Spotify's duration_ms anchors the match.
0003-two-layer-tool-error-convention	Tools propagate to hooks.py for a structured recovery payload rather than each wrapping try/except; retrieve_tracks_from_brain_state's inline DeapFileMissingError catch is the one sanctioned exception, because the hook payload drops the actionable message.
0004-subject-median-label-binarization	DEAP Likert labels split at each subject's own median rather than the literature's >= 5, which skews ~25/75 and bakes rater bias into LOSO folds; --label-split fixed_5 reproduces the paper setting.

Citations added to: the pgvector migration, models/track_audio_embedding.py, services/audio_catalog.py, agents/hooks.py, agents/tools/retrieval_tools.py, ml/dataset.py.

Migrations were touched in comments only — no migration code changed.

Part B — rules + AGENTS.md sync

• .claude/rules/conventions.md — docs/adr/ exists now; the "doesn't exist yet / until then" paragraph is replaced with the four records and how to add the next one.
• .claude/rules/frontend/vercel-ai-sdk.md — replaces the stale "panels switch on part.type, add a branch" bullet with the registry reality from Frontend tool-panel registry + feature slices #67 (slice map + one entry, hideRawOutput when the panel replaces raw JSON).
• .claude/rules/frontend/code-conventions.md — new "Feature slices" section: slice boundaries, no slice-to-slice imports, registry as the only aggregator, relative intra-slice vs @/ cross-directory.
• AGENTS.md — moved components now point at features/{sessions,retrieval}/, plus lines for the registry, the three-tier test layout, and docs/adr/. 116 lines, under the ~200 ceiling.
• Both frontend rules files now also match against frontend/features/**, which their paths: frontmatter predated.
• Backend code-conventions.md testing section was already rewritten in Adopt three-tier test layout with real-Postgres integration tier #66 — verified against the tree, unchanged.

Verification

• pre-commit run --all-files — passed (ruff, ruff-format, mypy)
• pytest (unit tier) — 175 passed
• No frontend source changed, so no lint run.

🤖 Generated with Claude Code