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

docs/project-summary.md

@@ -884,6 +884,24 @@ All pushed to `mysources.co.uk/michal/` registry.
 source .env && bash scripts/release.sh
 ```
 
+**The build refuses to run from a branch that is behind `main`.** Everyone
+branches off main, so a stale branch still builds and installs cleanly — it just
+ships a binary missing whatever landed on main meanwhile, and `rpm -U --force`
+overwrites the good one with it. That happened on 2026-08-10: a build from a
+stale checkout replaced `/usr/bin/mcpctl` with one that had no `statusline`
+command, months after the status line landed. `scripts/check-main-sync.sh`
+(sourced by `build-rpm.sh` and `build-deb.sh`) fetches `main`, compares, and
+fails before any work happens, listing the commits you are missing.
+
+```bash
+git merge main                                   # the fix
+MCPCTL_ALLOW_BEHIND_MAIN=1 bash scripts/release.sh  # deliberate old-tree build
+MCPCTL_BASE_BRANCH=release-2.x bash scripts/build-rpm.sh  # compare to another branch
+```
+
+Offline it falls back to the last fetched `origin/main`, then to a local `main`,
+and says which it used; outside a git checkout it skips entirely.
+
 Installs via nfpm:
 - `/usr/bin/mcpctl` — CLI binary (bun compiled)
 - `/usr/bin/mcpctl-local` — Local proxy binary (bun compiled)