fix: derive branch live from git, skip stale PRs

[htcarr3]

What

Two fixes for stale data in workspace status displays:

1. Remove Branch from stored workspace state — derive it live from git
2. Validate PR ancestry before showing PR badges

Why

Stale branch names: Branch was set once at workspace creation and never updated. Renaming a branch with git branch -m left the TUI, ws list, ws status, and MCP tools showing the wrong name.

Stale PR badges: gh pr view <branch> matches by name, not commit history. Reusing a branch name after a squash merge would incorrectly show the old merged PR.

How

Branch removal

• Removed Branch string from state.Workspace struct
• All callsites now call git.CurrentBranch(ws.Path) to get the live branch
• Added Branch field to TUI's workspaceItem for derived (non-stored) data
• JSON output structs retain "branch" in their contract, populated from git
• In archive, branch is captured before worktree removal
• Migration: Go's JSON decoder silently ignores unknown fields, so old state files with "branch" load fine

PR ancestry check

• PRStatus now fetches headRefOid from the PR
• Runs git merge-base --is-ancestor <head-sha> HEAD to verify the PR's head commit is in the branch's history
• Returns nil for name collisions (e.g. branch reused after squash merge)

Test plan

• bin/ci passes (build, vet, test, lint, tidy)
• Manual: fr8 dashboard shows correct branch; rename a branch, refresh, confirm it updates
• Manual: merge a PR (squash), create new branch with same name, confirm old PR badge is hidden