feat(phase-11): add workspace long-term memory

[lyxnbclass]

What changed

• add a workspace-local MemoryStore with bounded, atomic JSON persistence
• add deterministic phrase/tag/token retrieval without network or runtime dependencies
• expose search_memory, permission-gated save_memory, and permission-gated delete_memory tools
• add interactive /memory add|list|search|forget commands and cross-session recall
• include memory storage in offline readiness checks and document the complete stage design

Why

Stage 11 selects one advanced capability—long-term memory and retrieval—without bundling MCP, multi-agent orchestration, model routing, or TUI work. Durable project facts and explicit user preferences can now survive individual sessions while remaining scoped to one workspace.

Safety and behavior

• memory contents are never eagerly injected into the system prompt
• retrieved text is marked as untrusted, possibly stale historical data
• model-initiated writes and deletes reuse the fail-closed permission boundary
• Headless mode denies memory mutations unless --allow-mutations is explicit
• storage rejects corrupt schemas, cross-workspace copies, capacity overflow, and symlink escape
• generic file tools remain unable to access .minicode-rebuild/

Validation

• 275 passed, 3 skipped
• branch coverage: 85.45% (minimum 85%)
• Ruff passed
• Mypy passed for 28 source files
• compileall passed
• sdist and wheel builds passed
• no-network MockModel demo passed

The three local skips are Windows symlink-permission cases; the Ubuntu CI jobs execute those real escape-path tests.