Propagate build description through all log lines

[yushan8]

Every log line now carries the full request context, making it possible to filter logs by a specific request in production.

Problem: The request fields (build_description, first_revision, second_revision) were attached to a context logger at the top of each RPC handler, but most
subsequent log calls — including inside compareTargetGraphs, compareTargetGraphsAndEdges, and throughout the orchestrator — used c.logger/b.logger directly,
silently dropping that context.

Changes:

• orchestrator/native_orchestrator.go: Creates a context logger with build_description at the start of GetTargetGraph and uses it for all subsequent calls.
  Removes the per-call repetition of zap.Any("request build description", ...) (also fixes the inconsistent field name). Adds stage-level info logs: processing
  start, checkout done, requests applied, cache hit/miss, and computation complete.
• controller/getchangedtargets.go and getchangedtargetsandedges.go: All log calls inside the RPC handlers now use the context logger (carrying
  first_revision/second_revision) instead of the bare c.logger. The compareTargetGraphs and compareTargetGraphsAndEdges helpers now accept a *zap.Logger parameter
  (replacing the unused ctx parameter) so their logs also carry the request context.
• core/workspace/gitrequest.go: Adds a logger to gitRequest (inherited from the orchestrator's context logger, so it carries build_description). Each step of
  Apply() — fetch, diff, apply patch, commit, submodule update — now logs on start and on failure with request_id, base_ref, and commit.
• core/workspace/workspace.go: The Checkout warn log now includes remote and ref as structured fields