🤖 fix: reduce git lock conflicts in background polling (#915)

## Problem

Background git polling runs `git fetch` every 3-60 seconds. Even when
there's nothing new to fetch, `git fetch` acquires `maintenance.lock`,
which can conflict with:
- Manual git commands in terminal
- Agent git operations during streaming

## Solution

Check if fetch is needed before acquiring any locks:

1. `git ls-remote` → get remote SHA (network only, **no lock**)
2. `git cat-file -e` → is SHA already local? (**no lock**)
3. If local → skip fetch (**no lock needed**)
4. If not local → fetch to get new commits (lock, but only when
genuinely needed)

## Why this works

```
# Before: always locks
git fetch  →  maintenance.lock (even if nothing to fetch)

# After: only locks when needed  
ls-remote + cat-file  →  no lock (common case: SHA already local)
git fetch             →  lock (rare: new remote commits)
```

In practice, IDEs fetch periodically and agent `git pull` operations
keep refs current, so the "already local" path triggers most of the
time.

---

_Generated with `mux`_

Author: ammar-agent

src/common/utils/git/gitStatus.ts

@@ -82,10 +82,17 @@ export function parseGitStatusScriptOutput(output: string): ParsedGitStatusOutpu
 }
 
 /**
- * Optimized git fetch script with no prompts.
+ * Smart git fetch script that minimizes lock contention.
  *
- * Environment variables disable all interactive prompts (keychain, SSH, credentials).
- * Git flags optimize for speed - only fetch refs, not objects.
+ * Uses ls-remote to check if remote has new commits before fetching.
+ * This avoids locks in the common case where remote SHA is already local
+ * (e.g., IDE or user already fetched).
+ *
+ * Flow:
+ * 1. ls-remote to get remote SHA (no lock, network only)
+ * 2. cat-file to check if SHA exists locally (no lock)
+ * 3. If local: skip fetch (no lock needed)
+ * 4. If not local: fetch to get new commits (lock, but rare)
  */
 export const GIT_FETCH_SCRIPT = `
 # Disable ALL prompts
@@ -94,7 +101,29 @@ export GIT_ASKPASS=echo
 export SSH_ASKPASS=echo
 export GIT_SSH_COMMAND="\${GIT_SSH_COMMAND:-ssh} -o BatchMode=yes -o StrictHostKeyChecking=accept-new"
 
-# Fast fetch with optimization flags
+# Get primary branch name
+PRIMARY_BRANCH=$(git symbolic-ref refs/remotes/origin/HEAD 2>/dev/null | sed 's@^refs/remotes/origin/@@')
+if [ -z "$PRIMARY_BRANCH" ]; then
+  PRIMARY_BRANCH=$(git remote show origin 2>/dev/null | grep 'HEAD branch' | cut -d' ' -f5)
+fi
+if [ -z "$PRIMARY_BRANCH" ]; then
+  PRIMARY_BRANCH="main"
+fi
+
+# Check remote SHA via ls-remote (no lock, network only)
+REMOTE_SHA=$(git ls-remote origin "refs/heads/$PRIMARY_BRANCH" 2>/dev/null | cut -f1)
+if [ -z "$REMOTE_SHA" ]; then
+  echo "SKIP: Could not get remote SHA"
+  exit 0
+fi
+
+# Check if SHA exists locally (no lock)
+if git cat-file -e "$REMOTE_SHA" 2>/dev/null; then
+  echo "SKIP: Remote SHA already local"
+  exit 0
+fi
+
+# Remote has new commits we don't have - fetch them
 git -c protocol.version=2 \\
     -c fetch.negotiationAlgorithm=skipping \\
     fetch origin \\