feat: add auto-context recall to UserPromptSubmit hook

[blackaxgit]

Summary

• Add automatic context recall to the UserPromptSubmit hook — on every user prompt, the hook runs hybrid search (semantic + FTS5) against stored snapshots and injects top-K relevant results as additionalContext
• Extract shared RecallEngine in clx-core used by both MCP clx_recall tool and the hook, eliminating code duplication
• Add AutoRecallConfig with 8 tuneable fields, env var overrides, and dashboard Settings UI (9th section)
• Add SQLite busy_timeout(5s) for safe multi-session concurrency

Changes

File	Change
clx-core/src/config.rs	AutoRecallConfig struct, defaults, apply_f32_override, 8 env overrides
clx-core/src/recall.rs	NEW — RecallEngine, RecallHit, RecallQueryConfig, hybrid_merge, format_recall_context
clx-core/src/lib.rs	pub mod recall
clx-core/src/storage/migration.rs	busy_timeout(5s) in configure_pragmas()
clx-core/src/embeddings.rs	busy_timeout(5s) in EmbeddingStore::new() and with_dimension()
clx-core/src/storage/tests.rs	test_busy_timeout_is_set
clx-core/tests/integration.rs	test_concurrent_storage_access (multi-thread write test)
clx-hook/src/types.rs	prompt: Option<String> field on HookInput
clx-hook/src/hooks/subagent.rs	build_recall_context + do_recall with timeout, graceful degradation
clx-mcp/src/tools/recall.rs	Refactored to delegate to RecallEngine
clx/src/dashboard/settings/	Auto Recall as 9th section (sections, fields, config_bridge)
README.md	Document auto-recall feature and configuration

Key design decisions

• Hybrid search: semantic weight 0.6 + FTS5 weight 0.4, dedup by snapshot_id
• FTS5-first query order: FTS5 runs first (<10ms) before slow Ollama embedding, ensuring results within 500ms timeout
• Graceful degradation: Ollama down → FTS5 fallback; any error → orchestrator context only
• 500ms timeout: wraps entire recall operation; OllamaClient gets timeout−50ms with 0 retries
• UTF-8 safe: floor_char_boundary() for all string truncation
• Multi-session safe: busy_timeout(5s) on all SQLite connections prevents SQLITE_BUSY errors
• No OnceLock: hook is short-lived, direct construction avoids lifetime issues

Test plan

• cargo test --workspace — all tests pass (597 total)
• cargo clippy --workspace -- -D warnings — zero warnings
• Unit tests: score formula, hybrid merge dedup/weights/sorting/top-k, format budget, UTF-8 safety, early returns
• Unit test: test_busy_timeout_is_set verifies PRAGMA returns 5000ms
• Integration test: test_concurrent_storage_access — 2 threads, 20 writes each, zero SQLITE_BUSY
• Manual: hook returns recall context within default 500ms timeout (3/3 runs)
• Deployed locally and verified with real Ollama + FTS5 data

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