Make workspace authoring converge through immutable Nix candidates

Coordinate registered resource edits bottom-up into retained exact remote sources.
Use one Nix-owned source graph for provisional checking, template publication,
explicit baseline upgrades and host activation; retain independent runtime pins.

Add scoped contract inspection, historical recovery, derived worklists, crash-safe
locks, named dependency adoption and plain-QX structural editing. Repair TODO
ownership and template instantiation, and document the supported agent workflow.

Validated with protocol and command suites, real jj/Nix convergence and cache
checks, TS/React installed-command acceptance, and fresh TODO first-edit acceptance.
No live deployment or public publication performed. Props projection generation
and a one-command rich feature generator remain explicitly outside this delivery.

src/capability-language/file-lock.ts

@@ -0,0 +1,21 @@
+import { spawn } from "node:child_process";
+
+/** Kernel-owned lock: a crashed coordinator cannot leave a stale ownership file.
+ * The persistent file is just an inode; EOF releases the helper's lock. */
+export async function withFileLock<T>(filename: string, work: () => Promise<T>): Promise<T> {
+  const child = spawn("flock", ["--exclusive", "--nonblock", "--conflict-exit-code", "75", filename,
+    process.execPath, "-e", 'process.stdout.write("locked\\n"); process.stdin.resume();'], {stdio: ["pipe", "pipe", "pipe"]});
+  let diagnostics = "";
+  child.stdin.on("error", () => { /* acquisition/exit handling reports helper failure */ });
+  child.stderr.on("data", chunk => { diagnostics = (diagnostics + String(chunk)).slice(-2000); });
+  const closed = new Promise<void>((resolve) => { child.once("close", () => resolve()); });
+  try {
+    await new Promise<void>((resolve, reject) => {
+      let output = "";
+      child.once("error", reject);
+      child.once("exit", code => reject(new Error(code === 75 ? "Another authoring command owns this repository; retry when it finishes" : `Cannot acquire authoring lock: ${diagnostics}`)));
+      child.stdout.on("data", chunk => { output += chunk; if (output.includes("locked\n")) resolve(); });
+    });
+    return await work();
+  } finally { child.stdin.end(); await closed; }
+}