Publish final session summaries in PR descriptions

[d3cker]

PR descriptions currently repeat the bot's pre-work acknowledgement, while the completed session's actual results and tests remain only in the console. This change publishes the final successful assistant text with its Markdown preserved and no extra model summarization.

The dispatcher checkpoints each report with its session, round, verified commit and checks. Follow-ups retain the original report and replace one Latest update section after pushing. Dispatcher checks are explicitly separate from agent-reported tests. Missing legacy reports are recovered from saved sessions or clearly marked unavailable; analysis is never used as a fallback.

Description updates preserve manual notes outside stable HTML markers, reconcile lost responses without duplicate sections, and block on edits to the managed section or mismatched PR heads. Existing completed/closed tasks are not bulk rewritten. A second read detects concurrent edits before PATCH, but the read/write pair is not atomic. Large reports are explicitly truncated in the rendered body; full snapshots remain saved.

Documentation: updated README, architecture, runtime and recovery references, affected workflow diagrams (validated all eight), AGENTS source map, bundled bot final-report instructions, and Unreleased notes.

Validation:

• npm run check: lint, typecheck, 195 tests, native TUI render and build passed.
• npm run package:check -- /tmp/pr-summary-package: isolated package installation passed.
• Mermaid parser: all eight diagrams passed; 86 local documentation links checked; git diff --check passed.

Targets devel. No version bump or server deployment is included.