fix(regen): report missing dependencies as REGEN_DEPS_MISSING, not REGEN_STALE

When package.json declares dependencies and node_modules is missing, the
scratch worktree runs the generator without them. A generator that degrades
and still exits 0 then looked stale, and the fix line sent people to rerun
the generator. gstack-regen check and the pregate regen check now report
"dependencies missing; run <install>" with its own result code and
troubleshooting anchor; the install command follows the lockfile. The
fixture reproduces the shape GSTA-31 saw in gbrain.

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

docs/pregate.md

@@ -41,7 +41,9 @@ paths, at most 64 per list); commands are one line under 512 bytes.
   <paths>` for entries whose inputs or outputs intersect) in a scratch
   `git worktree` seeded from the working tree, with `node_modules` linked rather
   than copied, and reports stale outputs (`REGEN_STALE`, exit 1) without touching
-  the tree. An entry with no `command` is tracked only (goldens regenerated by
+  the tree. When outputs differ but `package.json` declares dependencies and
+  `node_modules` is missing, the entry is `REGEN_DEPS_MISSING` (install, then
+  rerun) instead of stale. An entry with no `command` is tracked only (goldens regenerated by
   another entry).
 - `gstack-regen write` regenerates in place: what `gstack-restamp` and the
   Step 3 conflict rule use. On a conflict in a registered output, take either

docs/troubleshooting.md

@@ -2717,6 +2717,18 @@ take either side, then regenerate.
 worktree has no `node_modules` of its own, so a command that needs them must
 reference the repository's (gstack's entries do through `bun run`).
 
+<a id="regen-deps-missing"></a>
+### `REGEN_DEPS_MISSING`
+
+**Meaning.** A registered command exited 0 and its outputs differ from the
+committed ones, but `package.json` declares dependencies and the repository has
+no `node_modules`, so the scratch worktree ran the generator without them. The
+difference may come from the missing dependencies rather than stale files, so
+the entry is reported as unverified, not `REGEN_STALE`.
+
+**Fix.** Run the printed install command (`bun install`, or the one matching the
+repository's lockfile), then rerun `gstack-regen check` or the pre-gate.
+
 <a id="pregate-config-invalid"></a>
 ### `PREGATE_CONFIG_INVALID`