Remove or mount the dead sandbox router in the API service

[priscaenoch]

Problem

src/api/sandbox.ts contains stub endpoints (breakpoint / continue) whose own documentation states: "sandbox router is not currently mounted in router.ts". Meanwhile the live sandbox routes the frontend uses (/api/sandbox, /api/sandbox/:id, /api/sandbox/simulate, /api/sandboxes) are served by the indexer. The src/ sandbox router is therefore dead code that duplicates, in stub form, functionality that already lives elsewhere.

What needs to be done

• Decide the sandbox's home. Since the frontend talks to the indexer, the indexer's sandbox routes should remain authoritative.
• Remove the unmounted src/api/sandbox.ts router and its stub endpoints, or, if it is intended to supersede the indexer's, mount it and implement it fully.
• Remove the stale "not mounted" documentation once resolved.

Files

• src/api/sandbox.ts
• src/api/router.ts

Acceptance deliverables

• No unmounted, stubbed sandbox router remains in the codebase.
• The single authoritative sandbox implementation is clear.

Tests to pass

• Build and existing tests pass after removal/mounting.