fix(cli): tolerate fts errors in search

[Steve-too]

What changed

Makes vouch search tolerate FTS5/index failures the same way the context, MCP, and JSONL retrieval surfaces already do:

• auto backend falls through to substring search when index_db.search() raises sqlite3.Error
• hybrid backend degrades to semantic-only fusion when FTS5 raises
• adds CLI regressions for both paths

Why

The CLI search path had a resilience gap: server/JSONL/context callers already guard FTS5 failures, but the CLI could still surface a raw search failure instead of returning the best available fallback results.

Tests

• PYTHONPATH=src python3 -m pytest tests/test_cli.py -q
• uv run --with ruff python -m ruff check src tests
• uv run --with mypy --with types-PyYAML --with jinja2 python -m mypy src

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[Steve-too]

Retargeted this PR to test and rebuilt the branch on top of origin/test, matching CONTRIBUTING.md. The diff remains limited to src/vouch/cli.py and tests/test_cli.py. Targeted local check passed: PYTHONPATH=src python3 -m pytest tests/test_cli.py -q.