refactor: Comment on Claude review failures in PR workflow

[ainsleyclark]

Motivation

• Ensure the CLAUDE code-review step can fail without leaving contributors uninformed by posting a visible failure note on the pull request.
• Provide the failure reason in a collapsible <details> accordion so the PR timeline stays concise while exposing useful debugging info when needed.

Description

• Add a Prepare Claude review failure comment step to .github/workflows/pr.yaml that builds a comment_body output containing an HTML marker and a <details> accordion with the best-available error text, conditional on steps.claude-review.outcome == 'failure' and github.event_name == 'pull_request'.
• Add a Comment on PR step that uses the repository action ./.github/actions/pr-comment to post or update the failure comment on the PR, passing the prepared comment_body and the PR number.
• Mirror the same two steps in the generator template internal/templates/.github/workflows/pr.yaml.tmpl using {{ ghExpr }} and {{ ghSecret }} helpers so generated projects receive identical behaviour.
• Use an HTML comment marker <!-- claude-review-failure --> to enable idempotent updates/minimisation of previous failure comments.

Testing

• Ran pnpm build, which failed due to Corepack being unable to download pnpm because of a proxy/403 error (network restriction).
• Ran go run ../../main.go update, which failed because the Go 1.25.3 toolchain download was forbidden by the network proxy.
• Ran go fmt ./..., which failed for the same Go toolchain download restriction.
• Ran go test ./..., which failed for the same Go toolchain download restriction.

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.94%. Comparing base (7f6b060) to head (b190854).
⚠️ Report is 496 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #410      +/-   ##
==========================================
+ Coverage   64.59%   69.94%   +5.35%     
==========================================
  Files         154      185      +31     
  Lines        6064     7394    +1330     
==========================================
+ Hits         3917     5172    +1255     
+ Misses       2064     2025      -39     
- Partials       83      197     +114     

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