feat(agent): HDF5Memory::search with source filters, re-ranking, confidence

`HDF5Memory::search(query_embedding, query_text, &SearchOptions)` is the
store's full search path. `SearchOptions::new(k)` is plain hybrid search
with the tuned default fusion; each further stage is opt-in:

- `with_sources([..])`: only records from these source channels. The
  filter applies before ranking, so a filtered search still returns up
  to k results, normalised over what it can return. The HNSW pool is
  over-fetched in proportion to what the filter removes, and the allowed
  records are scanned exactly whenever that costs fewer distance
  evaluations than the index would (~pool x M) — and as the fallback if
  the pool comes back short. Keyword matches are filtered too.
- `with_rerank(ReRankConfig)` re-ranks a max(3k, 10) candidate pool by
  relevance, recency, source authority and activation;
  `with_confidence(ConfidenceConfig)` drops low-confidence results;
  `at_time(now)` pins the recency clock.

These were reachable only through the OpenClaw backend, which is now
`search` with both on. Its Hebbian boost now goes to the k results it
returns rather than the whole 3k candidate pool. `hybrid_search` and
`hybrid_search_with` are wrappers and unchanged (tested bit for bit).

Measured on tank (search_harness --options-study --full, 3 runs): at
100K every filter — 50%, 10%, 1% of the store, and records far from the
query — returns the exact filtered top 10, and none is slower than an
unfiltered search (1%: 2.3 ms vs 4.6 ms). Re-rank + confidence costs
about 3%. A first version decided between index and exact scan by pool
size vs store size; it measured 0.976 recall at 12.3 ms on the
far-from-query filter, which is why the rule compares costs instead.

Tests: tests/search_options.rs (filter correctness and full pages via
both paths, far-from-query fallback, edge cases, equality with
hybrid_search_with, re-rank recency, confidence, boost scope).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-agent/src/lib.rs

@@ -72,6 +72,7 @@ use ephemeral::{EphemeralConfig, EphemeralStore};
 pub use ephemeral::{EphemeralEntry, EphemeralStats};
 use knowledge::KnowledgeCache;
 use memory_strategy::{Exchange, MemoryStrategy, StrategyOutput};
+pub use search::SearchOptions;
 use session::SessionCache;
 
 // --- Error type ---