fix(augment): BM25 floor for the rescue's FTS fallback (#2092)

[sloemo01]

What

Fixes #2092. The zero-result grep rescue's FTS fallback returned the first row without checking its relevance score, so a multi-token pattern where only one common token matched produced a confident "Wiki suggests ..." line for an unrelated file (repro: bluetooth codec negotiation fallback riding on "fallback" at BM25 3.45).

Root cause

_rescue in augment_cmd/search.py returned the first FTS row pointing at a code page with no score check. Weak matches cluster ~3; genuine ones score 2-3x higher (observed 3.45 vs 8.70), so the fallback leg had no quality gate while the symbol path held an exact-match bar.

Changes

• Added _RESCUE_FTS_MIN_SCORE = 6.0 — below the floor, silence beats a confident wrong suggestion.

Tests

• Below-floor silent, above-floor fires — sabotage-verified (below-floor test fails without the fix).

Note: test_plugin_content is red on main itself (v0.47.0 release bug, unrelated to this PR).

[RaghavChamadiya]

Flagging this before anyone merges it on green CI, because the direction is one I already ruled out on #2092.

An absolute BM25 floor cannot be a single constant across both backends. SQLite negates the FTS5 rank and it is unbounded (search.py:685); PostgreSQL uses ts_rank, which is 0-1 (search.py:774). A _RESCUE_FTS_MIN_SCORE tuned on SQLite silently kills the rescue fallback on every Postgres install, including hosted, and it fails closed: no error, just no results, which is the failure mode the rescue exists to prevent.

What #2092 needs is token coverage, not a score threshold, reusing the _pattern_terms guard already at search.py:132. That is dialect-independent by construction.

There is also a claim question. #2092 is held for @punk-dev-robot, who has the local before-and-after measurements and asked for it first; @Aman-goel-04 asked on 3 September and was declined for the same reason. Please check the issue thread before opening a PR against an assigned issue, and if you want the token-coverage version, say so on #2092 and let punk-dev-robot answer.

Closing nothing here, and I am not going to close it over my head, but I am not merging this shape. Not in today's release either way.