build: refuse to package from a branch that is behind main

Everyone branches off main and builds from their own branch. A branch that is
behind main still builds and installs perfectly — it just packages a binary
missing whatever landed on main meanwhile, and `rpm -U --force` overwrites the
good one with it. Nothing reports an error; the release succeeds and the feature
simply vanishes from the installed CLI.

That is what happened today: a build from a stale checkout replaced
/usr/bin/mcpctl with one that has no `statusline` command at all, months after
the status line landed on main (`mcpctl statusline` -> "unknown command").

`check-main-sync.sh` fetches main, compares, and fails before any work happens,
listing the commits the branch is missing and the merge that fixes it. Sourced
by build-rpm.sh and build-deb.sh — both are run standalone, so neither can rely
on the other having checked.

A hard failure rather than a warning: a warning scrolls past in a build log, and
the whole point is to stop before the artifact exists.
MCPCTL_ALLOW_BEHIND_MAIN=1 is the escape hatch for a deliberate old-tree build;
MCPCTL_BASE_BRANCH retargets the comparison. Offline it degrades to the last
fetched origin/main, then a local main, saying which it used; outside a git
checkout it skips.

Verified: passes on this branch (up to date with main); against a synthetic ref
one commit ahead, build-rpm.sh aborts with exit 1 before ensure_build_deps and
before any compilation; the escape hatch bypasses it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019wUmrfkVQR6CKcYKxENq7k

scripts/build-deb.sh

@@ -19,6 +19,11 @@ source "$SCRIPT_DIR/arch-helper.sh"
 resolve_arch "${MCPCTL_TARGET_ARCH:-}"
 # Sets: NFPM_ARCH, BUN_TARGET, ARCH_SUFFIX
 
+# Same guard as build-rpm.sh: this script is also run on its own, so it cannot
+# rely on that one having checked.
+source "$SCRIPT_DIR/check-main-sync.sh"
+check_main_sync
+
 # Check and install missing build dependencies
 source "$SCRIPT_DIR/ensure-deps.sh"
 ensure_build_deps