GitHub plugin: picking a PR in the thread sidebar should persist the thread↔PR link

[Andrew-Brevard]

Summary

In the thread's right sidebar, the GitHub panel says "No pull request is linked to this thread yet — pick one". Picking a PR there shows it, but the choice is never saved. Switching sidebar tabs (or reopening the thread) drops back to the picker, and the thread stays unlinked.

Current behavior (github plugin 0.2.1)

• The picker's onPick only sets component state in the thread panel; nothing is persisted.
• pullForThread resolves a thread's PR only from:
  1. the environment's branch PR (sdk.environments.pullRequest), or
  2. a link:pr:<repo>#<n> KV record, which is written only by addLink when a thread is spawned from Send agent / Review with agent.
• So an existing thread, or a thread without a git branch (e.g. a personal-workspace environment), can't be linked to a PR at all.

Requested

• Picking a PR in the thread panel should persist a link (reuse addLink with the current threadId), so it survives tab switches and reloads and shows up in the PR's linked threads.
• A way to unlink or change the linked PR from the same panel.
• Optionally, a CLI equivalent, e.g. bb github link <repo>#<number> [--thread <id>].

Steps to reproduce

1. Open any thread whose environment branch has no PR.
2. Open the GitHub tab in the right sidebar and pick a PR.
3. Switch to another sidebar tab, then back.
4. The picker shows again; the PR isn't linked.

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Classification: Bug · Priority Medium · Effort Medium · ui, github.

Report: Reproduction and root cause · REPRODUCED, high confidence · reproduction label confirmed-repro.

Root cause: The thread panel picker changes local state without saving a thread-to-PR link, so remounting loses the selection.

PR: None. A complete fix needs a decision on how manual links override and unlink branch-derived PR associations; a save-only patch would leave change and unlink ineffective.