fix(github): skip progress-comment edits whose visible content is unchanged

[aparajon]

Why this matters

A stagnant apply re-edits its PR progress comment every 30 seconds forever, even though every render is byte-identical except for two re-stamped timestamps (the "Last updated" footer and the attribution line). Each of those PATCHes spends the installation's shared GitHub write budget — under the secondary content-creation limit, a handful of stalled long-running applies is real pressure — while changing nothing a reader can see.

What it does

• Adds templates.CommentContentKey, which collapses rendered timestamps out of a comment body. Two renders of an unchanged status produce equal keys; any real change (rows, state, checksum) produces a different key. Durations such as the revert-window countdown survive in the key, so comments where the passage of time is the information keep editing live.
• The observer remembers the content key it last wrote per GitHub comment ID and skips edits whose key is unchanged. A cache miss (e.g. the handler-posted initial comment) always edits — uncertainty degrades to an extra edit, never a missed one.
• A comment younger than 10 minutes always edits, even when unchanged. Young comments are the ones people actively watch — apply just started, setup phase, first rows copying — and an early stall must not read as SchemaBot going silent. The grace cost is bounded (a stagnant young comment refreshes at most every 30s for 10 minutes), and rotation-created comments get a fresh window since a new comment means someone just acted.
• Past the grace window, the "Last updated" footer doubles as a liveness signal, so skips lapse after a 10-minute heartbeat: a stalled apply's comment still refreshes a few times an hour to show SchemaBot is watching, instead of freezing indefinitely.

tick → render → content key
        comment <10m old ──── edit (grace: always fresh while watched)
        key unchanged  ─┬─ written <10m ago → skip (no PATCH)
                        └─ written ≥10m ago → edit (liveness refresh)
        key changed    ──── edit immediately

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR reduces unnecessary GitHub API write traffic by avoiding progress-comment edits when the rendered status is unchanged aside from auto-stamped timestamps, while still ensuring periodic “liveness” updates.

Changes:

• Added templates.CommentContentKey to normalize rendered comment bodies by collapsing render-time timestamps into a stable comparison key.
• Updated CommentObserver to cache the last written content key per GitHub comment ID and skip PATCH edits when the key is unchanged, with a 10-minute heartbeat to refresh the “Last updated” footer.
• Added unit and integration coverage validating both skip behavior and heartbeat refresh behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
pkg/webhook/templates/common.go	Adds timestamp-collapsing CommentContentKey used to compare rendered comment “visible content”.
pkg/webhook/templates/common_test.go	Unit tests for CommentContentKey stability across re-renders and sensitivity to real progress changes/countdowns.
pkg/webhook/comment_observer.go	Adds last-written caching and heartbeat-based skip logic to avoid redundant GitHub comment edits.
pkg/webhook/apply_comment_integration_test.go	Integration tests verifying identical renders are skipped and heartbeat refreshes still occur.

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