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)

Changes

File	What
models.py	SYNC_METADATA_PATTERN, SyncMetadata, parse_sync_metadata(). Updated DEFAULT_BODY_TEMPLATE with metadata comment. Added src_commit_ts param to format_body
git_ops.py	get_commit_timestamp(), get_pr_body(), has_open_pr(), close_pr()
cmd_copy.py	Thread commit_ts through call chain. Pre-check in _push_and_pr to skip body update. Close stale PR in _sync_destination at total == 0
cmd_dep_update.py	Close stale PRs for NO_CHANGES results in _update_and_validate
models_test.py	Tests for parse_sync_metadata and format_body metadata roundtrip

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