Move PR review commands from `gh api` to `gh pr` namespace

[rowan-stein]

Change request: move PR review commands to gh pr

Requested by user: relocate the new review helper commands under the gh pr command group (not under api).

Scope

• Relocate/rename commands as follows while preserving functionality:
  • gh pr review open (GraphQL)
  • gh pr review add (GraphQL)
  • gh pr review submit (GraphQL)
  • gh pr see-comments (REST)
  • gh pr reply-comment (REST)
• Keep existing behavior, flags, outputs and error handling defined in Issue Add PR review commands: list comments, reply by comment id, and pending review flow #16.
• Documentation updates: README examples and help output.
• Backwards compatibility: add deprecation stubs for previous api commands that print a clear message (and optionally forward) for a grace period.

Acceptance Criteria

• New commands available under gh pr with the same flags/behavior.
• Old api commands emit a deprecation notice that points to the new location.
• Unit tests updated and green for the moved commands (go test ./pkg/cmd/... for touched packages); document any unrelated test caveats.
• Manual smoke test using a temporary PR, mirroring Issue Post-merge verification: PR review commands (see-comments, reply-comment, review open/add/submit) #18 plan (open/add/submit → see-comments → reply-comment; plus pending-review edge-case) and post results in this issue.

References

• Original spec: Add PR review commands: list comments, reply by comment id, and pending review flow #16
• Implementation merged: feat: add gh api review helper commands #17
• Post-merge verification: Post-merge verification: PR review commands (see-comments, reply-comment, review open/add/submit) #18

Notes

• Ensure we don’t collide with any existing upstream gh pr subcommands.
• Maintain consistent JSON output schemas.