feat: tiered show/snippet lookup fast paths

[SutuSebastian]

Summary

• Add fast-tier equality lookup for show / snippet when the query is an exact symbol name (positional name, optional kind/in, or lone name:Token without wildcards) via new covering index idx_symbols_name_covering.
• Route substring LIKE, multi-field, and FTS queries on the unchanged slow tier; behavior-preserving — same result rows, faster SQL for the common exact-name case.
• Retire docs/plans/tiered-lookup-fast-paths.md and lift tier guidance to architecture, roadmap, glossary, CLI/MCP help, agent-content, and find-symbol-definitions recipe.

Test plan

• bun test src/application/show-search-mode.test.ts
• bun test src/application/show-engine.test.ts (EXPLAIN asserts USING COVERING INDEX)
• bun test src/db.test.ts
• bun run typecheck / lint / format:check on touched files
• CI green

[changeset-bot]

🦋 Changeset detected

Latest commit: b97193a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@stainless-code/codemap	Patch

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 25306b5f-b80e-4e43-a879-f8daee554e98

📥 Commits

Reviewing files that changed from the base of the PR and between b509e4c and b97193a.

📒 Files selected for processing (22)

• .agents/rules/verify-after-each-step.md
• .changeset/tiered-lookup-fast-paths.md
• docs/architecture.md
• docs/glossary.md
• docs/plans/tiered-lookup-fast-paths.md
• docs/roadmap.md
• lint-staged.config.js
• src/application/http-server.test.ts
• src/application/mcp-server.test.ts
• src/application/mcp-server.ts
• src/application/show-engine.test.ts
• src/application/show-search-mode.test.ts
• src/application/show-search-mode.ts
• src/cli/cmd-show.ts
• src/cli/cmd-snippet.ts
• src/db.test.ts
• src/db.ts
• templates/agent-content/mcp-instructions.md
• templates/agent-content/rule/00-full.md
• templates/agent-content/skill/10-recipes-context.md
• templates/agent-content/skill/40-query-patterns.md
• templates/recipes/find-symbol-definitions.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/tiered-lookup-fast-paths

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