feat(telemetry): instrument context builder tool runs

[cameroncooke]

Summary

Instruments context-builder runs with coarse phase timing and shape metrics.

Stack: 5/10 · depends on PR 2 · Refs GH-183

What this changes

• ContextBuilderSentryTelemetry.swift (new) — wraps runs in a context_builder.run transaction with context_builder.discovery / export / oracle.response / selection.commit child spans; started/completed/failed breadcrumbs.
• MCPContextBuilderToolProvider.swift — call sites for the phase spans.

Business rules

• Records token_budget_bucket (bucketed, never a raw token size), result_count/selection_file_count, and cache_hit.
• No file paths, names, or contents — the typed model has no case that could carry them.

Key decisions

• Phase spans (discovery/export/oracle/selection) give workflow timing without exposing what was discovered or selected.

Test plan

• Builds in both modes. ✅ verified.

[cameroncooke]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• build(telemetry): generate and upload dSYMs in the release pipeline #332
• feat(telemetry): instrument Codex runtime events and session persistence #331
• feat(telemetry): instrument Agent Mode run lifecycle #330
• feat(telemetry): instrument workspace actions #329
• feat(telemetry): instrument file, prompt, selection and apply-edits tools #328
• feat(telemetry): instrument context builder tool runs #327 👈 (View in Graphite)
• feat(telemetry): instrument MCP tool dispatch and bootstrap admission #326
• feat(telemetry): start telemetry and trace app launch #325
• feat(telemetry): add typed Sentry telemetry model and bootstrap #324
• build(telemetry): link Sentry conditionally and document privacy model #323
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.