feat(copy): skip PR body overwrite when already synced from newer commit

[EspenAlbert]

Summary

• Embed source commit timestamp as <!-- path-sync: sha=... ts=... --> HTML comment in PR body to track which source state was synced
• Skip PR body overwrite when existing PR was already synced from an equal or newer source commit
• Close stale PRs when path-sync copy or dep-update finds zero changes (source and destination already in sync)
• Add keep_pr_on_no_changes config to prevent auto-closing stale PRs (on both SrcConfig and DepConfig)
• Add force_resync config to bypass the "PR already synced from newer commit" skip check (on SrcConfig only)

Design decisions

• Source commit timestamp (src_repo.head.commit.committed_datetime) rather than destination sync commit, because the destination commit is always created during the run and would defeat comparison
• HTML comment <!-- path-sync: ... --> is invisible in rendered markdown, backward-compatible (missing comment = always overwrite)
• Keep remote branch on close: next sync reuses the same branch name, no orphan risk
• Bool flags default to False: keep_pr_on_no_changes and force_resync preserve existing behavior when omitted
• force_resync only on SrcConfig: dep-update always resets to origin/default and has no timestamp-based skip