Add source string index for code-like literals

[Lee-take]

Summary

• add a source_strings side-table plus FTS5 index for compact code-like string literals such as route keys, collection names, event names, and config keys
• index exact literal sites during extraction, attaching each hit to its smallest enclosing symbol and file:line location
• expose searchSourceStrings() for direct literal lookups, and use source-string hits as entry points for query, explore, CLI, and MCP search/context paths
• preserve exact literal semantics for single code-like strings while allowing multi-term FTS lookup, and keep natural-language context ranking stable by only seeding source-string results for code-like queries
• replace rows on sync/delete/clear so the side-table stays aligned with the graph

This is the larger follow-up to the source-string use case discussed on #451: route paths, Directus collection names, RN/Unity bridge event names, and similar cross-repo string contracts are not symbols, but they still need a queryable source surface.

Validation

• npx vitest run __tests__/source-strings.test.ts __tests__/context.test.ts __tests__/context-ranking.test.ts __tests__/mcp-tool-allowlist.test.ts __tests__/security.test.ts — 87 passed, 2 skipped
• npm run build
• CLI smoke test against a temp TypeScript project: init, query '/live-scoring/append-event', and explore 'unity.score.updated'