Export benchmark patch artifacts

[sethkarten]

Summary

• add a benchmark patch artifact exporter that writes patch.diff, metadata.json, summary.md, and optional trajectory.jsonl
• capture tracked and untracked git diffs plus commit, branch, file lists, session stats, tests, failures, and final status metadata
• mark dependency/setup churn as rejected_setup_churn unless explicitly allowed
• expose the exporter through AgentSession, agent connections, /export patch [dir], and prime-agent daemon patch-artifact <session> [dir]

Testing

• npm --prefix packages/coding-agent test -- patch-artifact.test.ts
• ./node_modules/.bin/tsgo --noEmit
• npm run check
• isolated daemon smoke with temp socket/session dirs: create git repo, create session, export daemon patch-artifact, validate patch and metadata

Note

Add patch-artifact export command for benchmark patch artifacts

• Adds patch-artifact.ts, a new module that exports createPatchArtifact, which captures git diffs (tracked and untracked), writes patch.diff, metadata.json, summary.md, and optionally trajectory.jsonl to an output directory.
• Adds a patch-artifact <session> [dir] CLI subcommand to the daemon client and an /export patch [dir] interactive slash command variant.
• Extends the AgentConnection interface and all implementations (daemon, in-process, deferred) with exportPatchArtifact(outputDir?), and adds a corresponding export_patch_artifact daemon protocol message.
• Filters setup/dependency churn files (e.g. lockfiles, node_modules) by default, reporting them as rejectedFiles; set allowSetupChurn to include them.

^{Macroscope summarized f8c719f.}

[sethkarten]

Closing as out of scope for the autonomous-mode work. Patch extraction is not needed for this path right now; #222 covers autonomous continuation and gate-driven terminal evidence.