Auto-post follow-up cross-link comment on originating issue

[sebastientaggart]

Adds a single cross-link comment to the originating issue when /submit-for-review's Step 9 follow-up flow creates one or more new tickets. The comment lists each follow-up by number and title so a reader of the originating issue's thread can see the trailing work without digging into the merged PR's review comment.

Gate conditions (skip silently otherwise):

• One or more follow-up tickets were actually created in Step 9.
• A linked originating issue was identified in Step 3.

Comment format:

## Follow-up tickets from PR #<merged-pr>

The code review on the PR for this issue surfaced non-blocking items tracked separately:

- #<f1> — <title1>
- #<f2> — <title2>

Repeated invocations of /submit-for-review on the same originating issue accumulate distinct comments (each referencing its own PR) — no deduplication or amending.

Single-file source edit; four adapter outputs regenerated.

Closes #176

[sebastientaggart]

Review Summary

Verdict: APPROVE

Findings

• [NOTE] Single-source edit in skills/github-agile/submit-for-review.md with four adapter regenerations (.agents, .claude, .cursor, .gemini), consistent with the project's sync-from-source convention.
• [NOTE] The new step references CodeCannon/skills/github-agile/scripts/post-issue-comment.py (present in the repo) and reuses the same <tmpdir> and Step 3 originating-issue lookup already used elsewhere in the skill, so no new infrastructure is required.
• [NOTE] Gate conditions in the doc ("follow-ups created AND originating issue identified, else skip silently") match the PR description, and the "no deduplication/accumulate" behavior is explicitly stated.