Improve PR review instructions

[eshulman2]

Summary

• improve the informational comment Forge posts directly on newly created GitHub pull requests
• explain how to submit review feedback through a Request changes review
• clarify that Forge automatically addresses submitted review feedback
• clarify that regular PR comments remain available for conversation and Forge commands

Testing

• uv run pytest tests/unit/workflow/nodes/test_pr_creation_informational_comment.py (3 passed)
• uv run ruff check src/forge/workflow/nodes/pr_creation.py tests/unit/workflow/nodes/test_pr_creation_informational_comment.py

[eshulman2]

🤖 Reviewing a Forge pull request

To send code feedback to Forge, open Files changed, select Review changes, and submit a Request changes review. Forge will address the review feedback automatically and push updates to this pull request.

Regular comments are ignored by the review workflow. Use them for conversation or the Forge commands below:

• /forge rebase - Merge the base branch (e.g. main) into this PR branch, with conflicts resolved by AI.
• /forge skip-gate <name> - Skip a named CI check (substring match) for this PR. This setting persists across subsequent pushes.
• /forge unskip-gate <name> - Remove a previously set CI check skip.