Code maps vertical slice for curated context

[jamesrochabrun]

Parent: #398

Depends on: Context Builder MVP and token estimates.

Goal

Add a first, narrow code-map implementation that lets users include signature-only API context for large files at much lower token cost.

Implementation Notes

Do not attempt the full multi-language RepoPrompt code-map system in one issue. Start with a vertical slice, preferably Swift first because AgentHub itself is Swift-heavy.

Suggested first slice:

• signature/API model for one language
• content fingerprint cache
• per-file mode in Context Builder: full / code map / exclude
• assembled <file_map> section for mapped files
• token estimate uses code-map text instead of full file text
• unsupported languages degrade to full/exclude without crashing

Tree-sitter may be used, but isolate dependencies so the rest of AgentHub does not pay unnecessary build complexity. If adding grammars, include license attribution for vendored grammar dependencies.

Acceptance Criteria

• A large Swift file can be added as code map and contributes signatures without function bodies.
• Code-map token estimate is materially lower than full-file estimate.
• Re-running on unchanged content uses the cache.
• Unsupported files do not crash context assembly.

Tests

Add tests for extraction output, cache invalidation by content fingerprint, fallback behavior, and assembly into the <file_map> section.