docs(federation): M3 mission planning — 14-task decomposition + manifest update

Decomposes Milestone 3 (mTLS handshake + list/get/capabilities + scope
enforcement) into 14 tasks following the M1/M2 pattern. Updates mission
manifest to reflect M2 done, M3 in-progress (2/7 milestones complete),
and appends session 23 entry to the MVP scratchpad.

M3 structure:
- Foundation: M3-01 (DTOs in packages/types/src/federation/)
- Server stream: M3-03 (AuthGuard) → M3-04 (ScopeService) → M3-05/06/07 (verbs)
- Client stream (parallel): M3-08 (FederationClient) → M3-09 (QuerySourceService)
- Test infra (parallel): M3-02 (tools/federation-harness/ — local two-gateway)
- Validation: M3-10 (Integration) → M3-11 (E2E) → M3-12 (Independent security review)
- Close: M3-13 (Docs) → M3-14 (release tag fed-v0.3.0-m3, close #462)

Estimate ~100K tokens vs MILESTONES.md 40K — same per-task expansion as M1/M2
once tests, review, and docs are split out.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/scratchpads/mvp-20260312.md

@@ -612,3 +612,44 @@ Independent security review surfaced three high-impact and four medium findings;
 7. DEPLOY-03/04 acceptance probes (`mosaic gateway doctor --json`, pgvector `vector(3)` round-trip)
 8. DEPLOY-05: author `docs/federation/TEST-INFRA.md`
 9. M2-02 (Step-CA sidecar) kicks off after image health is green
+
+### Session 23 — 2026-04-21 — M2 close + M3 decomposition
+
+**Closed at compaction boundary:** all 13 M2 tasks done, PRs #494–#503 merged to `main`, tag `fed-v0.2.0-m2` published, Gitea release notes posted, issue #461 closed. Main at `4ece6dc6`.
+
+**M2 hardening landed in PR #501** (security review remediation):
+
+- CRIT-1: post-issuance OID verification in `ca.service.ts` (rejects cert if `mosaic_grant_id` / `mosaic_subject_user_id` extensions missing or mismatched)
+- CRIT-2: atomic activation guard `WHERE status='pending'` on grant + `WHERE state='pending'` on peer; throws `ConflictException` if lost race
+- HIGH-2: removed try/catch fallback in `extractCertNotAfter` — parse failures propagate as 500 (no silent 90-day default)
+- HIGH-4: token slice for logging (`${token.slice(0, 8)}...`) — no full token in stdout
+- HIGH-5: `redeem()` wrapped in try/catch with best-effort failure audit; uses `null` (not `'unknown'`) for nullable UUID FK fallback
+- MED-3: `createToken` validates `grant.peerId === dto.peerId`; `BadRequestException` on mismatch
+
+**Remaining M2 security findings deferred to M3+:**
+
+- HIGH-1: peerId/subjectUserId tenancy validation on `createGrant` (M3 ScopeService work surfaces this)
+- HIGH-3: Step-CA cert SHA-256 fingerprint pinning (M5 cert handling)
+- MED-1: token entropy already 32 bytes — wontfix
+- MED-2: per-route rate limit on enrollment endpoint (M4 rate limit work)
+- MED-4: CSR CN binding to peer's commonName (M3 AuthGuard work)
+
+**M3 decomposition landed in this session:**
+
+- 14 tasks (M3-01..M3-14), ~100K estimate
+- Structure mirrors M1/M2 pattern: foundation → server stream + client stream + harness in parallel → integration → E2E → security review → docs → close
+- M3-02 ships local two-gateway docker-compose (`tools/federation-harness/`) so M3-11 E2E is not blocked on the Portainer test bed (which is still blocked on `FED-M2-DEPLOY-IMG-FIX`)
+
+**Subagent doctrine retained from M2:**
+
+- All worker subagents use `isolation: "worktree"` to prevent branch-race incidents
+- Code review is independent (different subagent, no overlap with author of work)
+- `tea pr create --repo mosaicstack/stack --login mosaicstack` is the working PR-create path; `pr-create.sh` has shell-quoting bugs (followup #45 if not already filed)
+- Cost tier: foundational implementation = sonnet, docs = haiku, complex multi-file architecture (security review, scope service) = sonnet with two review rounds
+
+**Next concrete step:**
+
+1. PR for the M3 planning artifact (this commit) — branch `docs/federation-m3-planning`
+2. After merge, kickoff M3-01 (DTOs) on `feat/federation-m3-types` with sonnet subagent in worktree
+3. Once M3-01 lands, fan out: M3-02 (harness) || M3-03 (AuthGuard) → M3-04 (ScopeService) || M3-08 (FederationClient)
+4. Re-converge at M3-10 (Integration) → M3-11 (E2E)