fix(version): suppress staleness warning for fork builds

When the gt binary is built from a local fork (e.g., ~/src/gastown),
the staleness check would incorrectly warn about being stale because
it compared against the rig's repo HEAD. This confused agents.

The fix detects fork builds by checking if the binary's embedded
commit exists in the target repo. If not, the binary was built from
a different repo and we report "fork build" instead of "stale".

Changes:
- Add IsForkBuild field to StaleBinaryInfo
- Add commitExistsInRepo helper to detect fork scenarios
- Update gt stale command to show "Binary built from fork" status
- Update doctor check to report fork builds as OK

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/version/stale.go

@@ -21,6 +21,7 @@ var (
 // StaleBinaryInfo contains information about binary staleness.
 type StaleBinaryInfo struct {
 	IsStale       bool   // True if binary commit doesn't match repo HEAD
+	IsForkBuild   bool   // True if binary was built from a different repo (fork)
 	BinaryCommit  string // Commit hash the binary was built from
 	RepoCommit    string // Current repo HEAD commit
 	CommitsBehind int    // Number of commits binary is behind (0 if unknown)
@@ -66,6 +67,15 @@ func commitsMatch(a, b string) bool {
 	return strings.HasPrefix(a, b[:minLen]) || strings.HasPrefix(b, a[:minLen])
 }
 
+// commitExistsInRepo checks if a commit hash exists in the given repository.
+// This is used to detect fork builds - if the binary's commit doesn't exist
+// in the target repo, the binary was likely built from a different repo.
+func commitExistsInRepo(commit, repoDir string) bool {
+	cmd := exec.Command("git", "cat-file", "-t", commit)
+	cmd.Dir = repoDir
+	return cmd.Run() == nil
+}
+
 // CheckStaleBinary compares the binary's embedded commit with the repo HEAD.
 // It returns staleness info including whether the binary needs rebuilding.
 // This check is designed to be fast and non-blocking - errors are captured
@@ -93,6 +103,14 @@ func CheckStaleBinary(repoDir string) *StaleBinaryInfo {
 	// Compare commits using prefix matching (handles short vs full hash)
 	// Use the shorter of the two commit lengths for comparison
 	if !commitsMatch(info.BinaryCommit, info.RepoCommit) {
+		// Check if the binary's commit exists in this repo.
+		// If it doesn't, the binary was built from a different repo (fork).
+		// Don't warn about staleness in that case.
+		if !commitExistsInRepo(info.BinaryCommit, repoDir) {
+			info.IsForkBuild = true
+			return info
+		}
+
 		info.IsStale = true
 
 		// Try to count commits between binary and HEAD