feat: Cherry-pick PR author attribution and optional assignee

[rnetser]

Summary

Cherry-pick PRs currently lose the original author's identity in label-triggered flows and cannot be assigned to the original PR author.

Problem / Motivation

When cherry-pick PRs are created, they have two issues:

1. Missing author attribution in label-triggered cherry-picks: When a cherry-pick is triggered by a cherry-pick-<branch> label (applied before the original PR is merged), the PR description says requested-by by target-branch label. This loses the identity of who opened the original PR.

2. PR ownership: Cherry-pick PRs are always created as the bot user (whoever owns the GitHub token). There's no way to assign the cherry-pick PR to the original PR author.

Requirements

1. When cherry-pick is triggered by label (on merge), the description should say requested-by by <original-pr-user> with target-branch label instead of requested-by by target-branch label
2. Add a new config option cherry-pick-assign-to-pr-author (boolean, default: false) at both global and per-repository level
3. When enabled, the cherry-pick PR should be assigned to the original PR author using hub's -a flag

Deliverables

• Update cherry_pick() method in runner_handler.py to accept by_label flag and use original PR user
• Update pull_request_handler.py to pass original PR user when calling cherry_pick() from label path
• Add cherry-pick-assign-to-pr-author config option to schema.yaml (global + per-repo)
• Add config reading in github_api.py
• Implement assignee logic in cherry_pick() hub command
• Update tests
• Update examples/config if needed

Notes

The cherry-pick flow has two trigger paths:

• Comment-triggered (/cherry-pick <branch>): Already knows the commenter's username
• Label-triggered (cherry-pick-<branch> label on merge): Currently loses the original PR author identity