chore: Archive completed task files (M0, M1, M3, M3.5, M4.1-2, M3.6.1)

Deleted 31 completed task files:
- M0.x: 8 tasks (cargo, domain types, recordsource, tokenizer, adapters, gate)
- M1.x: 8 tasks (llm-chat, standing-query, prompt template, parser, loop, log, e2e, gate)
- M3.x: 4 tasks (l2-synthesis, rerank, mem-query, gate)
- M3.5.x: 8 tasks (http-server, ingest, query, federation, skills, projects, rate-limiting, gate)
- M3.6.1: DocCorpusSource (heading-boundary chunking)
- M4.1-2: skill-draft, derived-filter

Updated INDEX.md:
- Removed M0 & M1 phase sections (archived in git history)
- Updated progress table: 65 active tasks (42✅ + 2🟡 + 21⬜)
- Updated status: M0/M1 complete, M3/M3.5 gates passing, M4.1-2 done
- Noted M3.5.10 JWT auth implementation complete (awaiting image rollout)
- Cleaned up broken links to deleted task files

Total test count: 239 passing, 2 ignored (up from 196 at M3.4)
Ready for M4.3 gate composition, M5 post-training, M7 source connectors.

tasks/M8.2-dual-write-indexer.md

@@ -0,0 +1,83 @@
+# M8.2 — Dual-write indexing pipeline
+
+| Field | Value |
+|---|---|
+| Phase | M8 — Hybrid Search |
+| Size | M — 1–2 days |
+| Status | ⬜ |
+| Flags | — |
+| Spec | inlined below |
+| Blocks | M8.4, M8.5 |
+| Depends | M8.1 (OpenSearch running), M2.4 (pgvector repo) |
+
+## Goal
+
+When a document is ingested, write to **both** pgvector (embedding) and OpenSearch (raw text) atomically. Same `chunk_id` in both stores. If one write fails, log error but don't block the other — eventual consistency, not transactions.
+
+## Design
+
+**Unified ID mapping:** Both stores use the same `chunk_id` (UUID). The ingest worker generates the ID once, writes to both.
+
+**Chunking policy:** 512-token chunks with 10% (51-token) overlap. Deterministic — same input always produces same chunks with same IDs.
+
+**Dual write sequence:**
+1. Chunk document (heading-boundary or fixed-size).
+2. Generate embedding via LLM.
+3. Write to pgvector: `INSERT INTO chunks (id, embedding, text, source, project, level, breadcrumb)`.
+4. Write to OpenSearch: `PUT vault-{project}/_doc/{chunk_id}` with `{content, source, level, breadcrumb, project_id, indexed_at}`.
+5. If OpenSearch write fails: log warning, mark chunk as `opensearch_pending=true` in pgvector. Background retry later.
+
+**OpenSearch index mapping:**
+```json
+{
+  "content": {"type": "text", "analyzer": "standard", "boost": 2.0},
+  "section_title": {"type": "text", "boost": 1.5},
+  "breadcrumb": {"type": "keyword"},
+  "source": {"type": "keyword"},
+  "project_id": {"type": "keyword"},
+  "level": {"type": "keyword"},
+  "indexed_at": {"type": "date"}
+}
+```
+
+**Deduplication:** Before writing, check `chunk_hash` (SHA256 of text). If hash exists and `is_indexed=true` in both stores, skip.
+
+## Steps
+
+1. Add `opensearch_pending` boolean column to `chunks` table (migration).
+2. Update `IngestWorker::process_ingest()` to call OpenSearch after pgvector write.
+3. Make `OpenSearchClient::index_document()` public, fix method signature.
+4. Add background task: retry `opensearch_pending=true` chunks every 5 minutes.
+5. Add dedup check before dual write.
+
+## Acceptance
+
+1. `mem ingest --dry-run` on a test doc shows chunks written to both stores.
+2. Same `chunk_id` exists in both `SELECT id FROM chunks` and `GET vault-*/_doc/{id}`.
+3. Kill OpenSearch mid-ingest: pgvector write succeeds, chunk marked `opensearch_pending=true`.
+4. Restart OpenSearch: background retry picks up pending chunks within 5 minutes.
+5. Re-ingest same document: dedup skips already-indexed chunks (0 new writes).
+
+## Verify
+
+```bash
+# Ingest a test document
+cargo run -- ingest --project test --source fixtures/refcorpus/small.md
+
+# Check pgvector
+psql -c "SELECT id, source, opensearch_pending FROM chunks WHERE project='test'"
+
+# Check OpenSearch
+curl -k -H "Authorization: Bearer $TOKEN" \
+  https://opensearch-internal:9200/vault-test/_search | jq '.hits.total'
+
+# IDs must match
+```
+
+**False pass:** Both stores have data but with different IDs — the join on `chunk_id` finds zero matches. Assert `SELECT count(*) FROM chunks WHERE id IN (opensearch_ids)` equals total indexed.
+
+## Artifacts
+
+- Modified `crates/mem-cli/src/ingest_worker.rs`
+- Modified `crates/mem-store/src/lib.rs` (migration)
+- Modified `crates/mem-cli/src/opensearch_client.rs`