feat(ci sync): post a run summary on the pull request

[luvkapur]

Problem

A sync run can change what a lane component records — a snapped version, a dependency range, a drift fix — with no file in the branch's own commit ever changing. The pull request shows only the git diff. A reviewer looking at the PR has no way to see what the run actually did to the scope.

Fix

After a successful export, upsert one maintained comment on the branch's pull request: the components the run snapped (name and new version), and the synced pair's anchors (branch name and tip, lane id and head). "Upsert" — one comment, updated in place on every run, marked by a hidden marker, never a growing pile of one-off posts.

This needs a git host capability bit ci sync did not have: reading a PR's existing comments (paginated — GitHub returns 30 per page by default) and updating one by its marker instead of only ever posting new ones. GitHostProvider.upsertComment is optional: a provider that doesn't implement it is skipped, not faked with a plain comment call that would leave a new copy on every push.

No configured git host, no open PR yet, or a comment API failure are all normal states, not failures. The first two skip silently; the third only warns. None of them can undo an export or a git push that already succeeded.

Design notes — open for discussion

This PR is deliberately narrow: components snapped, plus the synced pair's anchors. Left open:

• What else belongs in the summary? New (never-exported) components, environment changes, and config synced down from main all change the scope without a source diff, same as a snapped version does.
• Comment vs. check-run. A PR comment is simple and needs no extra permissions, but a GitHub check-run would show the summary inline in the PR's checks tab instead of as a bot comment.
• Opt-out. Some teams may not want a standing bot comment on every PR. Should sync.runSummary (or similar) turn it off?
• merge-diverged. This PR wires the comment into export-branch only. The diverged-branch path runs the same snap-and-export and arguably deserves the same summary — held back here to keep the diff reviewable.

Tests

• github-client.spec.ts: upsertComment — posts when no marked comment exists, patches the marked one in place without a second post, honors createIfAbsent: false, and follows Link-header pagination to find a comment past page 1.
• lane-sync-executor.spec.ts: changedLaneComponents and runSummaryCommentBody as pure functions; postRunSummaryComment against a fake git host — present, absent, missing the optional method, no open PR, and a throwing upsertComment.
• e2e/harmony/ci-sync.e2e.ts, the lane <-> branch reconcile cycle (scenarios A–D, including the export-branch scenario this PR touches) — unaffected with no git host configured, matching this PR's silent-skip path.