refactor(server): effectify session summary route handlers

[Astro-Han]

Summary

Effectify the session diff and artifacts JSON handlers.

Why

Issue #936 is moving route handlers onto AppRuntime-backed services while preserving the existing HTTP API shape. This keeps the read-only session summary routes on SessionSummary.Service without touching summarize, prompt, share, revert, or permission behavior.

Related Issue

Fixes part of #936.

Human Review Status

Pending

Review Focus

Please focus on whether /:sessionID/diff and /:sessionID/artifacts preserve their existing response shape and remain limited to read-only summary routes.

Risk Notes

No visible UI or copy changed, so the UI checklist item is not applicable.
No platform, packaging, updater, signing, path, shell, or permissions surface was touched, so the platform checklist item is not applicable.
No docs, release notes, dependencies, permissions, credentials, deletion behavior, generated content, or local file surface changed beyond the focused route test update.

How To Verify

Focused route test: cd packages/opencode && bun test ./test/server/session-runtime-routes.test.ts -> 1 passed, 0 failed
Typecheck: cd packages/opencode && bun run typecheck -> passed
Diff check: git diff --check -> passed

Screenshots or Recordings

Not applicable: no visible UI changes.

Checklist

• Type label — this PR carries exactly one of bug, enhancement, task, documentation. Type labels are author-added; the labeler bot does NOT assign them. Add the label in the GitHub UI, then tick this.
• Routing labels — this PR carries at least one of app, ui, platform, harness, ci. The labeler bot assigns these on PR open based on changed paths. Confirm the bot's choice (or override if wrong), then tick this.
• Priority label — this PR carries exactly one of P0, P1, P2, P3. The priority-triage bot suggests one on PR open. Confirm or override, then tick this.
• Human Review Status above is set to Pending, Approved by @<reviewer>, or Not required: <reason> (default is Pending; "not required" is restricted to bot-authored low-risk PRs).
• I linked the related issue, or stated in Summary why there is no issue.
• I described the review focus and any meaningful risks.
• I replaced the example block in How To Verify with the real verification steps and the key result for each.
• I did not introduce unrelated refactors, dependencies, generated files, or file changes beyond the stated scope.
• (conditional) I manually checked visible UI or copy changes when needed, with screenshots or recordings. Leave unticked only if no visible UI or copy changed.
• (conditional) I considered macOS and Windows impact for platform, packaging, updater, signing, paths, shell, or permissions changes. Leave unticked only if no platform/packaging surface was touched.
• (conditional) I called out docs, release notes, dependencies, permissions, credentials, deletion behavior, generated content, or local file changes when relevant. Leave unticked only if none of those surfaces was touched.
• I reviewed the final diff for unrelated changes and suspicious dependency changes.
• I am targeting dev, and my PR title and commit messages use Conventional Commits in English.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: cd8e9b6b-e652-4c0c-a31a-82556fe751a6

📥 Commits

Reviewing files that changed from the base of the PR and between c019c11 and dccc57d.

📒 Files selected for processing (2)

• packages/opencode/src/server/instance/session.ts
• packages/opencode/test/server/session-runtime-routes.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/i936-session-summary-route-effect

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[github-actions]

Suggested priority: P2 (includes non-doc, non-test paths outside the low-risk bucket).

P1/P0 are reserved for maintainer confirmation. Please relabel manually if this is a release blocker, security issue, data-loss risk, or updater/runtime failure.

[gemini-code-assist]

Code Review

This pull request refactors the 'diff' and 'artifacts' session route handlers in 'session.ts' to run within the 'AppRuntime' using Effect generators and resolve 'SessionSummary.Service' dynamically. Additionally, it updates the integration tests in 'session-runtime-routes.test.ts' to include assertions for the '/session/:sessionID/diff' endpoint. No review comments were provided, so there is no additional feedback to address.