Extract formatNodeRef helper in webhook semantic/FTS renderers

Non-blocking maintainability nit from sage review of #93.

src/server/tana-webhook-server.ts repeats the same node-reference shape in three spots (semantic node, semantic ancestor, FTS):

if (name.includes("[[") && name.includes("]]")) {
  const match = name.match(/\[\[.+?\]\]/);
  ref = match ? match[0] : name.trim();
} else {
  ref = `[[${name}^${id}]]`;
}

Extract a local formatNodeRef(name: string, id: string): string and call it for node, ancestor, and the FTS renderer. Keeps the three branches from drifting. Also include the || "(unnamed)" null-coalesce inside the helper so the guard lives in one place.

Filed from #93 review, 2026-06-12.

Start in src/server/tana-webhook-server.ts and inspect the semantic node, semantic ancestor, and FTS renderer branches that build node references. Extract the shared formatting behavior, including the unnamed fallback, and verify that all three renderers retain their existing output without drifting.