Add optional body indexing with single-copy FTS5 storage

[DandyLyons]

Goal

Make body storage and full-text indexing opt-in while retaining efficient, single-copy FTS5 search. Part of Phase 2.5 in #93; follows #137 and #138 and coordinates with #139 and #141.

Approved design

• Persist one indexing mode per index initially, avoiding conflicting policies across overlapping scopes.
• New indexes default to metadata-only: persist metadata, provenance, memberships and diagnostics, but neither bodies nor an FTS index. Do not require an FTS5 capability probe in this mode.
• Existing indexes retain their search capability during migration unless explicitly changed.
• FTS mode retains exactly one cached body per document. Use an external-content FTS5 table referencing that copy, with correct transactional insert/delete/update triggers and stable row identity. A separate body table is not required; preserve the one-to-one metadata/body relationship.
• Retokenize only when the body changes, not when only metadata or assessments change.
• Define explicit configuration/CLI controls for enabling and disabling search, migration, rebuild, and removal of obsolete body/FTS storage. Document when disk space is reclaimed.
• Search requests in metadata-only mode fail clearly; do not silently enable indexing or substitute different search semantics.
• Files remain authoritative. Metadata-only mode is intended to compose with tools such as ripgrep; FTS mode provides indexed word/phrase/proximity search and ranking. Do not implement a competing regex engine.

Acceptance criteria

• New metadata-only indexes contain no persisted body payload or FTS tables and work without FTS5 support.
• Existing indexes migrate without unexpectedly disabling search.
• FTS mode stores one body copy and passes insert/update/delete/rebuild and FTS consistency checks.
• Metadata-only changes do not retokenize unchanged bodies.
• Mode switches, interruption, and failed migration have explicit tested behavior.
• Shared documents across scopes retain one body and correct independent membership.
• CLI/configuration/help and external SQLite query documentation describe both modes and unavailable-search errors.
• Pending edits are preserved through mode changes; coordinate with Add revision-checked index apply for pending file edits #142.

Boundaries

No mandatory ripgrep dependency or server subprocess integration. Core remains SQLite-independent. Streaming query output belongs to #139; server capability/OpenAPI behavior belongs to #141.

[DandyLyons]

Completed in 4800db3697735b73bf97c9b3318b153e1a341c75. Optional body indexing now defaults new caches to metadata-only, preserves FTS for legacy caches, stores one external-content FTS5 body copy, provides explicit search mode controls and unavailable-search errors, and avoids FTS5 probing when reopening metadata-only caches. Verification on 2026-09-17: swift test --filter 'MarkdownUtilitiesIndexTests|IndexCommandsTests' passed all 41 tests.