feat(v3.0): cross-project federated query (Phase 53.5)

[RichardHightower]

Summary

Implements cross-project federated query: a single query can span multiple registered project stores, with project attribution on each result.

Phase numbering note (rebased 2026-05-12): Originally drafted as v3.0 Phase 51 before the Retrieval Orchestrator (PR #28) took that slot. Rebased onto post-Phase-51 main and renumbered as Phase 53.5 — an out-of-band decimal phase, mirroring the Phase 51.5 pattern used for PR #27. This preserves v3.0 Competitive Parity's documented scope (Phases 51-53) without renaming planned work.

What changed

File	Change
proto/memory.proto	Added all_projects flag to RouteQueryRequest, project attribution field to RetrievalResult
memory-types/config.rs	New CrossProjectConfig { registered: Vec<PathBuf> } + projects field in Settings
memory-storage/db.rs	Added Storage::open_read_only() for federation
memory-service/federated.rs (NEW)	federated_query() — fans out to registered stores, merges results, project attribution, fail-open
memory-service/retrieval.rs	Wired federated_query behind all_projects opt-in flag
e2e-tests/cross_project_test.rs (NEW)	4 E2E tests
.planning/STATE.md, .planning/ROADMAP.md	Recorded as v3.0 Phase 53.5 (out-of-band)

Design decisions

• Opt-in only — default single-project behavior is completely unchanged
• Fail-open — unavailable stores are silently skipped, never block a query
• Project attribution — project field on results uses serde(default) for backward-compatible JSON, matching the existing agent field pattern from v2.1
• Index-free fallback — added a primary TOC fallback inside federated_query so cross-project works even when BM25/vector indexes aren't built for a store
• Config-driven — register project paths in config.toml under [projects]

Tests

13/13 passing:

• 9 unit tests (federated module)
• 4 E2E tests: merged results, project attribution, fail-open on unavailable store, default behavior unchanged

Verified via task pr-precheck after rebase: fmt + clippy + tests + docs all green.

Usage

```toml

~/.config/agent-memory/config.toml

[projects]
registered = [
"/path/to/project-a",
"/path/to/project-b"
]
```

Then query with all_projects: true in the RouteQueryRequest.