Diff badge stops counting work that already merged

[pallaoro]

After a squash merge done outside Ateam (an agent's gh pr merge), a task kept showing the whole branch as changes (+5465 on a task whose PR had merged) and its card said closed.

Why

• The badge counted git diff --merge-base origin/<base>. A squash or rebase merge never moves the merge-base, so merged lines counted forever.
• origin/<base> was never fetched after a merge outside Ateam.
• detectMerged asked about task.branch, but agents and loop runs cut fresh branches inside their worktree.
• When one file had both committed and unstaged edits, the unstaged count overwrote the committed one.

Change

• diff/fileDiff: files the branch no longer contributes (git merge-tree --write-tree, in memory, no index lock) count only edits on top of HEAD. Conflicted files count as landed when base held HEAD's exact version at some point since the fork. Older git, or no origin/<base>, keeps today's numbers.
• The sweep and the gitDiff RPC fetch origin/<base> first: once per repo per 5 min, 5s cap, reusing fetchBase.
• detectMerged follows the worktree's current branch.
• With a base set, one --merge-base pass already covers the working tree, so the staged/unstaged override is dropped.

Verified

• 4 new git-core tests; 3 fail on the old code. Disabling the history recheck brings back the created-file regression.
• Typecheck passes; server 357/357 and git-core 61/61 tests pass.
• Against the real worktrees: clawnify-integrations goes from +5465 to +54 (only the untracked .codex/), PR #2286 merged (was closed). openvideo goes to +0, #2288 merged.

Ceiling: committing again on a file after it was squash-merged falls back to the full merge-base count for that file.