[DSP-6] Code Intelligence: blast_radius, relevant_context, symbol_at, callers/callees/dependents from devswarm

[justrach]

Child of #53. Source: justrach/devswarm.

Tools to port (6)

DevSwarm tool	Purpose	Forge landing zone (proposed)
blast_radius	Files/symbols affected by a proposed change	forge_app + codedb integration
relevant_context	Locate sections related to a query/symbol	uses existing semantic search
symbol_at	Resolve symbol at file:line	forge_app
find_callers / find_callees	Forward/reverse call graph traversal	already partially in codedb (codedb_callers, codedb_deps)
find_dependents	Reverse dependency mapping	codedb

Note: heavy overlap with codedb (already present)

The repo already has codedb_* MCP tools that cover most of this surface. The work here is not to reimplement, but to:

1. Expose these as first-class SDK / TUI commands, not just MCP tools.
2. Wire them into the swarm path: the orchestrator (DSP-1) should call blast_radius to scope worker assignments, and the reviewer agent should call it before signing off on a fix.
3. Add a review_pr_impact aggregator (used by DSP-5).

Acceptance criteria

• blast_radius accepts a diff/commit and returns affected files + symbols + downstream callers.
• Schema validates non-empty args end-to-end, not just at the JSON layer (lesson from devswarm #429).
• Orchestrator uses blast_radius output to partition tasks across workers.
• Reviewer agent attaches blast_radius summary to its review output.