feat: link branch by URL and add get_task context hook

[pikann]

Summary

• Adds a "link branch" flow that accepts a pasted GitHub branch URL (or bare branch name) and links it to a task
• Improves branch-link error handling to surface 403 (insufficient token permissions) separately from 404 (branch not found)
• Adds a getToolContext hook for get_task so linked branches and pull requests are surfaced in AI tool context
• Bumps minCoreVersion to v0.12.2 and version to 0.3.3

Test plan

• Link a branch via pasted GitHub URL and verify it resolves the correct repo/branch
• Link a branch via bare branch name against a selected repo
• Verify 403/404/already-linked error states render correctly
• Verify get_task tool context includes linked branches and PRs