feat(ui): banner above commits when current branch is behind upstream

[gfargo]

Adds a one-line banner to the history view that announces "the remote has work you don't" for the current branch. Fills a real gap: the chip work in 0.52.0 made remote refs visually distinct when they appear in the row set, but on a behind branch the upstream-only commits aren't reachable from local HEAD, so chips alone never signal it.

What it looks like

Behind-only (current branch is N commits behind, no local commits ahead):

Commits *                                  143/143 commits | compact graph | loaded
↓ 2 commits behind origin/main · F fetch · U pull
●─  feat: foo                  [HEAD -> main]
●─  refactor: bar

Diverged (both ahead AND behind — fast-forward pull is impossible):

Commits *                                  145/145 commits | compact graph | loaded
↑2 ↓2 diverged from origin/main · F fetch · U pull --rebase
●─  feat: local-only-A         [HEAD -> main]
●─  feat: local-only-B

Design choices

Question	Choice	Why
Color	Warning yellow	Same semantic as the remote-tracking chip kind from 0.52.0
Behind-only wording	↓ N commits behind <upstream>	Matches git status for familiarity
Diverged wording	↑N ↓N diverged from <upstream>	Reuses the ↑N ↓N symbols from formatBranchDivergence
Pull hint when diverged	U pull --rebase	Fast-forward pull is impossible with local work; rebase keeps history linear
Show on ahead-only?	No	Nothing inbound to pull. Push affordances live on the branches sidebar
Show on synced / no-upstream / detached HEAD?	No	Nothing inbound. Detached HEAD has no currentBranch ref
Change row set / auto-fetch?	No	Pure chrome. Surface the affordance, not the action

ASCII fallback mirrors formatBranchDivergence: v for ↓, +N/-N for ↑N ↓N, . for ·, no color.

Implementation

• New helper formatUpstreamAheadBanner in src/workstation/chrome/iconography.ts (~30 LOC). Two variants, plural-correct, ASCII-fallback, returns undefined when there's nothing to surface.
• One render branch in src/workstation/surfaces/history/index.ts injecting the banner between the panel header and the existing filter indicator. Reads context.branches.localBranches.find(b => b.current) for the branch ref.
• No new actions / keybindings. The F and U hints point at the existing fetch-remotes and pull-current-branch global workflows.

Tests

12 new in iconography.test.ts:

• undefined branch (detached HEAD) → no banner
• no upstream → no banner
• behind === 0 (synced or ahead-only) → no banner
• behind-only: N commits, singular noun for behind === 1, arbitrary upstream names, ASCII fallback
• diverged: ↑N ↓N format, pull --rebase hint, ASCII fallback

Test plan

• npm run test:jest -- src/workstation src/commands/log — 985 pass (was 973 + 12 new)
• npm run lint — clean on touched files
• Manual: npm run scenario create branch-sync-showcase -- --run-ui — HEAD is on main which is 2 behind origin/main. Banner should read ↓ 2 commits behind origin/main · F fetch · U pull in warning yellow above the commit list. Press F or U and verify the existing workflows fire.
• Manual: from the same scenario, git checkout feat/diverged and re-run. Banner should read ↑2 ↓2 diverged from origin/feat/diverged · F fetch · U pull --rebase.
• Manual: git checkout feat/synced and re-run. No banner (synced).

Pairs with

• git-scenarios#4 (branch-sync-showcase v0.3.3) — the fixture this PR is designed against
• chore: bump @gfargo/git-scenarios to ^0.3.3 #1013 (coco dep bump to ^0.3.3) — landed first
• Upcoming task Add support for HuggingFace Interface #4 (branches sidebar iconography + per-branch hotkeys) — same scenario as fixture