fix: substitute {pr} in pr-review-reply.yaml posting command

[derekmisler]

Problem

review-pr/agents/pr-review-reply.yaml line 76 contains:

gh api repos/{owner}/{repo}/pulls/{pr}/comments --input -

{pr} is not a gh CLI template variable — only {owner} and {repo} are. This produces a 404 on every reply attempt, forcing the agent to retry with an explicit URL. The same root cause as the false double-post bug fixed in PR #111, but in the reply path.

posting-format.md is now sed-rendered by the 'Copy reference files' step (PR #111), but pr-review-reply.yaml is not — so every feedback reply still pays a guaranteed 404 + retry.

Fix

Either:

• Render pr-review-reply.yaml through the same sed substitution in 'Copy reference files', or
• Replace {pr} with $PR_NUMBER and ensure PR_NUMBER is in the agent's env at runtime

Tracked as a follow-up to PR #111.

[derekmisler]

Consolidated into PR #111 (commit a9d1f63). The fix adds a "Stage reply agent" step to review-pr/reply/action.yml that renders pr-review-reply.yaml to /tmp/pr-review-reply.yaml with {pr} substituted before the agent runs, using the same sed pattern as the posting-format.md fix.