meta: serialize all ops behind a tokio mutex + clear stale lock at startup

journal showed three concurrent rebuilds racing on the meta
repo's .git/index.lock — auto_update::run kicks off parallel
tokio::spawn for every stale agent, each rebuild eventually
calls into meta::sync_agents / lock_update_for_rebuild which
do git add + commit, git isn't safe across concurrent processes
on the same .git/, and one of the failing-mid-write children
left index.lock behind. subsequent ops blocked until somebody
rm'd it manually.

fix: static META_LOCK (tokio::sync::Mutex<()>) acquired at the
top of every public meta function. concurrent rebuilds take
turns on meta ops; the actual nix build (nixos-container update)
releases the lock first and runs without it, so parallel agent
builds still parallelize on nix-daemon's own concurrency model.

migrate::run additionally clears /var/lib/hyperhive/meta/.git/
index.lock on startup if it exists — we just booted, nothing
of ours is holding it. covers the 'previous crash left a stale
lock' case the user just hit so the daemon recovers without
manual intervention.

hive-c0re/src/meta.rs

@@ -20,6 +20,7 @@ use std::path::{Path, PathBuf};
 
 use anyhow::{Context, Result, bail};
 use tokio::process::Command;
+use tokio::sync::Mutex;
 
 use crate::lifecycle;
 
@@ -28,6 +29,17 @@ const APPLIED_ROOT: &str = "/var/lib/hyperhive/applied";
 const GIT_NAME: &str = "hive-c0re";
 const GIT_EMAIL: &str = "hive-c0re@hyperhive";
 
+/// Single-writer lock around every meta-repo operation. Git isn't
+/// safe to drive from concurrent processes against the same `.git/`
+/// — two simultaneous `git add` / `commit` invocations race on
+/// `.git/index.lock`; if either dies before releasing, the lock
+/// sticks and the next operation hits "another git process seems to
+/// be running" until somebody `rm`s it manually. Holding this mutex
+/// across each public function's git+nix calls makes parallel
+/// rebuilds (`auto_update` + dashboard-triggered + apply-commit)
+/// take turns instead of colliding.
+static META_LOCK: Mutex<()> = Mutex::const_new(());
+
 /// Where the manager sees this directory inside its container (RO bind).
 #[allow(dead_code)] // wired up by set_nspawn_flags in a follow-up commit
 pub const CONTAINER_MANAGER_META_MOUNT: &str = "/meta";
@@ -56,6 +68,7 @@ pub async fn sync_agents(
     operator_pronouns: &str,
     agents: &[AgentSpec],
 ) -> Result<()> {
+    let _guard = META_LOCK.lock().await;
     let dir = meta_dir();
     std::fs::create_dir_all(&dir).with_context(|| format!("create {}", dir.display()))?;
 
@@ -104,6 +117,7 @@ pub async fn sync_agents(
 /// meta history only carries successful deploys.
 #[allow(dead_code)] // wired up by actions::run_apply_commit in a later commit
 pub async fn prepare_deploy(name: &str) -> Result<()> {
+    let _guard = META_LOCK.lock().await;
     let dir = meta_dir();
     let input = format!("agent-{name}");
     nix(&dir, &["flake", "update", &input]).await?;
@@ -118,6 +132,7 @@ pub async fn prepare_deploy(name: &str) -> Result<()> {
 /// place (nothing staged → nothing to commit).
 #[allow(dead_code)]
 pub async fn finalize_deploy(name: &str, sha: &str, tag: &str) -> Result<()> {
+    let _guard = META_LOCK.lock().await;
     let dir = meta_dir();
     if !has_staged_changes(&dir).await? {
         return Ok(());
@@ -131,6 +146,7 @@ pub async fn finalize_deploy(name: &str, sha: &str, tag: &str) -> Result<()> {
 /// captured in `applied/<n>`'s annotated `failed/<id>` tag.
 #[allow(dead_code)]
 pub async fn abort_deploy() -> Result<()> {
+    let _guard = META_LOCK.lock().await;
     let dir = meta_dir();
     git(&dir, &["restore", "--staged", "flake.lock"]).await?;
     git(&dir, &["restore", "flake.lock"]).await
@@ -157,6 +173,7 @@ async fn has_staged_changes(dir: &Path) -> Result<bool> {
 /// semantics — it always wants the latest main.
 #[allow(dead_code)] // wired up by lifecycle::rebuild in this commit
 pub async fn lock_update_for_rebuild(name: &str) -> Result<()> {
+    let _guard = META_LOCK.lock().await;
     let dir = meta_dir();
     let input = format!("agent-{name}");
     nix(&dir, &["flake", "update", &input]).await?;
@@ -172,6 +189,7 @@ pub async fn lock_update_for_rebuild(name: &str) -> Result<()> {
 /// because the per-agent inputs aren't touched.
 #[allow(dead_code)]
 pub async fn lock_update_hyperhive() -> Result<()> {
+    let _guard = META_LOCK.lock().await;
     let dir = meta_dir();
     nix(&dir, &["flake", "update", "hyperhive"]).await?;
     if git_is_clean(&dir).await? {