feat(v3.0): cross-project unified memory

[RichardHightower]

Summary

Implements the first v3.0 feature: federated query across multiple project stores.

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	v3.0 Phase 51 progress recorded

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

Usage

# ~/.config/agent-memory/config.toml
[projects]
registered = [
  "/path/to/project-a",
  "/path/to/project-b"
]

Then query with --all-projects flag or all_projects: true in config.