docs(bin): state main as the default-branch convention, master as legacy fallback

bin/fm-merge-local.sh

@@ -44,7 +44,7 @@ default_branch() {
 BRANCH="fm/$ID"
 git -C "$PROJ" rev-parse --verify --quiet "refs/heads/$BRANCH" >/dev/null || { echo "error: branch $BRANCH does not exist in $PROJ" >&2; exit 1; }
 
-DEFAULT=$(default_branch) || { echo "error: cannot determine default branch for $PROJ; expected origin/HEAD, main, or master" >&2; exit 1; }
+DEFAULT=$(default_branch) || { echo "error: cannot determine default branch for $PROJ; expected origin/HEAD, or a local main (legacy master also accepted)" >&2; exit 1; }
 
 # The project's main checkout must be on its default branch and clean, so the
 # fast-forward lands predictably (firstmate never writes here otherwise).

bin/fm-review-diff.sh

@@ -65,7 +65,7 @@ default_branch() {
   return 1
 }
 
-DEFAULT=$(default_branch) || { echo "error: cannot determine default branch for $PROJ; expected origin/HEAD, main, or master" >&2; exit 1; }
+DEFAULT=$(default_branch) || { echo "error: cannot determine default branch for $PROJ; expected origin/HEAD, or a local main (legacy master also accepted)" >&2; exit 1; }
 
 BRANCH="fm/$ID"
 if ! git -C "$WT" rev-parse --verify --quiet "refs/heads/$BRANCH" >/dev/null; then

bin/fm-tangle-lib.sh

@@ -18,7 +18,7 @@
 # primary checkout is the alarm.
 
 # Resolve the default branch name of the git repo at <dir>: prefer origin/HEAD,
-# then fall back to a local main/master. Echoes the name, or returns 1.
+# then fall back to a local main (legacy master as a secondary fallback). Echoes the name, or returns 1.
 fm_default_branch() {
   local dir=$1 ref branch
   ref=$(git -C "$dir" symbolic-ref --quiet --short refs/remotes/origin/HEAD 2>/dev/null || true)

bin/fm-teardown.sh

@@ -1163,7 +1163,7 @@ validate_worktree_teardown_safety() {
   unpushed=$(printf '%s\n' "$unpushed_raw" | head -5)
 
   if [ -n "$unpushed" ] && [ "$MODE" = local-only ]; then
-    DEFAULT=$(default_branch) || { echo "REFUSED: cannot determine default branch for $PROJ; expected origin/HEAD, main, or master." >&2; return 1; }
+    DEFAULT=$(default_branch) || { echo "REFUSED: cannot determine default branch for $PROJ; expected origin/HEAD, or a local main (legacy master also accepted)." >&2; return 1; }
     if ! unmerged_raw=$(git -C "$WT" log --oneline HEAD --not "$DEFAULT" -- 2>/dev/null); then
       if worktree_safety_blocked_by_lock "commits not on $DEFAULT"; then
         return "$TEARDOWN_WORKTREE_SAFETY_LOCK_BLOCKED"

docs/architecture.md

@@ -165,7 +165,7 @@ Its operating checkout (`FM_ROOT`) and the disposable crewmate worktrees are all
 The primary checkout is healthy on its default branch, and linked worktrees or secondmate homes are healthy at detached HEAD.
 Only a named non-default branch checked out in `FM_ROOT` is a worktree tangle.
 
-`fm-tangle-lib.sh` resolves the default branch from `origin/HEAD`, then local `main` or `master`, and classifies that named non-default primary branch as the tangle.
+`fm-tangle-lib.sh` resolves the default branch from `origin/HEAD`, then local `main` (legacy `master` as a secondary fallback), and classifies that named non-default primary branch as the tangle.
 `fm-guard.sh` prints the repair command on the next mutable fleet action, while `bin/fm-session-start.sh` reports the same condition through bootstrap as a `TANGLE:` line at session start.
 If another live session holds the fleet lock, both surfaces keep the alarm but switch to read-only wording with no repair command.
 Ship briefs also tell the crewmate to verify `pwd -P` and `git rev-parse --show-toplevel` before creating `fm/<id>`, then stop with a blocked status if it landed in the primary checkout.