[PENDING] refactor(graphrunner): route metrics through observability/metrics

[xytan0056]

[PENDING] — do not merge until pr6-orchestrator lands

Routes the native graphrunner's metrics through observability/metrics.

• The graphrunner reads the request emitter from ctx via metrics.FromContext (the orchestrator forwards the same ctx the controller seeded), so it no longer takes a tally.Scope at construction.
• Substep timers (bazel_query_duration, git_file_hashes_duration, target_hash_duration) and the target-count gauge move to shared metric-name constants under the graph_runner op sub-scope. target_count → targets_count, still a gauge to preserve the time-series shape.
• With the runner taking its emitter from ctx, the orchestrator's Scope plumbing (Params.Scope + the threaded field) is now dead and removed.

Stacking / dependency

Based on pr6-orchestrator. Independent of #174. Same as pr6: the emitter is seeded by the controller PR (pr5); until then FromContext returns the noop emitter (safe).

[xytan0056]

Superseded by #198. GitHub auto-marked this PR as "merged" when the stack order was flipped so the graphrunner change lands before the orchestrator change (it couldn't be reopened). The graphrunner work now lives in #198 (based on pr4-context).