ci(post-release): name conflicting files in merge-failure Slack alert

[jason10lee]

All Submissions:

• Have you followed the Newspack Contributing guidelines?
• Does your code follow the WordPress coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

When the post-release release → alpha/release → main back-merge hits a conflict, the Slack alert previously said only that a merge failed and linked the build — leaving the reader to open the Actions log to find out what conflicted or who to involve. This makes the alert self-contained and routable.

The alert now names each conflicting file and, for each, the incoming release-side PR (as a clickable link) and its author, so a reader can tell what needs reconciling and who to ping straight from the message:

⚠️ Post-release merge to main failed for: Automattic/newspack-workspace.
Conflicting files:
• path/to/file — incoming: #000 (PR Author)
Check <…|the build> for details.

Attribution identifies the incoming change being merged forward (not sole blame — the merge is mutual), resolved from the squash commit that last touched each file.

Built to be robust on this already-failed path:

• Size-safe. The named list is capped (first 10 files + "…and N more"); a hard line-boundary trim backstops Slack's 3000-char section limit so a many-conflict merge can't produce an oversized payload that Slack rejects (which would drop the entire alert). The build link is always preserved.
• JSON-safe. The payload is built with node's JSON.stringify rather than hand-rolled string interpolation, correctly escaping the variable-length file list.
• Always notifies. If the node payload build fails, it falls back to a minimal hand-rolled alert instead of going silent; it never aborts the script (set -e) or leaves subsequent merges / status reporting unreported.
• Edge cases. Non-ASCII paths stay literal (core.quotePath=false) so attribution matches; PR/author parsing is robust to merge commits (--no-merges), tabs in commit subjects, and issue-vs-PR refs; gracefully degrades when a commit has no (#NNN).

When there are no conflicting paths to report, the message is unchanged from before.

How to test the changes in this Pull Request:

1. In a throwaway git repo, create a release branch and a default branch that both modify the same line of a file, so git merge release conflicts; give the release-side commit a (#NNN) subject.
2. Run post-release.sh (or exercise notify_slack / attribute_conflicts) with Slack env vars pointed at a test channel, and trigger the failing merge path.
3. Confirm the conflicting path is captured before git merge --abort, and that the posted message lists it under Conflicting files: with the incoming PR rendered as a clickable Slack link (<url|#NNN>) and the author name.
4. Confirm size-safety: with many (or pathologically long-pathed) conflicts, the rendered text stays ≤ 2900 chars, the file list is capped with "…and N more", and the build link survives (backticks stay balanced).
5. Confirm graceful degradation: a no-conflict failure path renders the previous (unchanged) message; a commit with no (#NNN) shows just the file (or author); a forced node failure still posts the minimal fallback alert.
6. Confirm the generated payload is valid JSON in the multi-file, no-conflict, and fallback cases.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable? (No test harness exists for post-release.sh; verified manually — bash -n, shellcheck, node payload generation across size/edge cases, and simulated conflicting merges.)
• Have you successfully run tests with your changes locally?

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