fix(release-agent): replace blocked git/gh commands with report_progress in Phase 3

[Copilot]

In the Copilot coding-agent sandbox, git push and gh pr create are blocked, making the old Phase 3 steps 8–9 non-functional.

Changes

• .github/agents/release.agent.md — In "Phase 3: Validate and Create PR":
  • Added a callout explaining the sandbox push constraint: report_progress is the only viable commit/push mechanism; a draft PR targeting main is pre-created by the Copilot system and does not need to be opened manually.
  • Replaced old steps 8–9 (shell-based git commit, git push, gh pr create) with revised steps 8–10 using report_progress to atomically commit + push, verify success via its output, and notify the user with post-merge reminders.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.3%. Comparing base (bb1cd9d) to head (e0df24f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1288     +/-   ##
=======================================
- Coverage   81.3%   81.3%   -0.1%     
=======================================
  Files        111     111             
  Lines       9377    9377             
=======================================
- Hits        7625    7624      -1     
- Misses      1752    1753      +1     

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