feat: helper locks GBRAIN_DATABASE_URL at startup, defends against config rewrites

The wireup helper previously read ~/.gbrain/config.json on every gbrain
subprocess invocation. On Garry's Mac, multiple concurrent test runs and
agent integrations were rewriting that file mid-sync, redirecting the
wireup at the wrong brain partway through a 4-min initial import.

This commit adds a `--database-url <url>` flag to the helper and locks
the URL at startup. Precedence:
  1. --database-url flag                       (explicit caller intent)
  2. GBRAIN_DATABASE_URL / DATABASE_URL env    (CI / manual override)
  3. read once from ~/.gbrain/config.json      (default)

Whichever wins gets exported as GBRAIN_DATABASE_URL for every child
`gbrain` invocation. Per gbrain's loadConfig at src/core/config.ts:53,
env-var URLs override the file URL — so a process that flips config.json
between two of our gbrain calls can't redirect us. Defense-in-depth:
once the URL is locked, the wireup completes against the original brain
even under hostile filesystem conditions.

setup-gbrain/SKILL.md.tmpl Step 7 now reads the URL out of config.json
once (via python3 inline) and passes it explicitly with --database-url,
so even the very first wireup call is decoupled from config.json mutability.

Three new test cases cover the lock behavior:
  - --database-url flag is exported to child gbrain calls
  - falls back to ~/.gbrain/config.json when no flag and no env
  - flag overrides env GBRAIN_DATABASE_URL and config.json values

The fake gbrain in the test suite now records GBRAIN_DATABASE_URL alongside
each call so tests can assert the helper exported the locked URL.

Total test count: 13 → 16 passing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -23,6 +23,8 @@ gbrain search "ethos" --source gstack-brain-{user} | head -5
 
 `bin/gstack-gbrain-source-wireup` is the new helper. It derives a per-user source id from `~/.gstack/.git`'s origin URL (with multi-fallback to `~/.gstack-brain-remote.txt` and a `--source-id` flag), creates a detached `git worktree` at `~/.gstack-brain-worktree/`, registers it as a federated source on gbrain, runs initial backfill, and supports `--strict` (Step 7 strictness), `--uninstall` (full teardown including future-launchd plist), and `--probe` (read-only state inspection). All idempotent. The helper depends on `jq` (transitive via `gstack-gbrain-detect`).
 
+The helper locks the database URL at startup (precedence: `--database-url` flag > `GBRAIN_DATABASE_URL`/`DATABASE_URL` env > read once from `~/.gbrain/config.json`) and exports it as `GBRAIN_DATABASE_URL` for every child `gbrain` invocation. This means external rewrites of `~/.gbrain/config.json` mid-sync (e.g., a concurrent `gbrain init --non-interactive` running in another workspace) cannot redirect the wireup at a different brain. Per gbrain's `loadConfig()`, env-var URLs override the file. Step 7 of `/setup-gbrain` reads the URL out of `config.json` once and passes it explicitly via `--database-url`, so the wireup is robust against config flips during the seconds-to-minutes sync window.
+
 `/setup-gbrain` Step 7 now invokes the helper with `--strict` after `gstack-brain-init`. `/gstack-upgrade` invokes the helper without `--strict` via `gstack-upgrade/migrations/v1.12.3.0.sh` so missing/old gbrain is a benign skip during batch upgrade. `bin/gstack-brain-restore` invokes the helper after the initial clone so a 2nd Mac gets the wireup automatically. `bin/gstack-brain-uninstall` invokes `--uninstall` plus removes legacy `consumers.json`.
 
 `bin/gstack-brain-init` drops 60 lines of dead consumer-registration code (the HTTP POST block, the `consumers.json` writer, the chore commit). `bin/gstack-brain-restore` drops the 18-line `consumers.json` token-rehydration block (the only consumer that used it never had real tokens). `bin/gstack-brain-consumer` is marked deprecated in its header docstring; removal in v1.13.0.0 after one cycle of grace.