The API's issue dependencies read names its Server-Timing phases: session, dependencies, render #4924

Intent: a slow read of /api/v1/repos/:owner/:name/issues/:number/dependencies says where its time went, as the commits and issues lists already do.

Done:

• [ ] GET /api/v1/repos/:owner/:name/issues/:number/dependencies answers a Server-Timing header naming session, dependencies and render
• [ ] the test fails if a phase is dropped

Follow test/api-branches-timing.test.ts: the handler wraps its reads in timePhase (src/core/route_timing.ts) and notes session with takeSessionTime, and a new test/api-deps-timing.test.ts asserts the Server-Timing header names each phase. Touch only this handler in src/http/api.ts, since other issues edit other handlers in parallel. The answer's body does not change.