feat: add dedicated API endpoint for focus/context slicing

[jonathanpopham]

Context

The focus command (internal/focus/handler.go) does inline graph traversal to extract a token-efficient context slice for a file — finding defines_function/defines and calls/contains_call edges. Unlike dead-code and blast-radius, there is no dedicated API endpoint for this.

Current state

• supermodel focus <file> uses the generic /v1/graphs/supermodel graph and traverses it locally
• Returns functions defined in the file, what they call, and what calls them

Proposal

Either:

1. API-side: Add POST /v1/analysis/focus that returns a pre-built context slice with richer data (types, imports, callers, token estimate)
2. CLI-side: Keep using the graph but improve the traversal (it currently misses many functions due to the defines_function relationship sparsity)

Blocked on

• Decision on whether this warrants an API endpoint or should remain client-side

[coderabbitai]

🔗 Related PRs

#23 - feat: wire dead-code to /v1/analysis/dead-code endpoint [closed]

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