docs: capture local Grav core upgrade + refresh version-authority docs

New learning: docs/solutions/tooling-decisions/upgrade-local-grav-core-rebuild-docker-image.md
— the local Grav core is baked into the Docker image (only ./user is bind-mounted),
so it upgrades by a Dockerfile URL bump + image rebuild + `docker rm -f` recreate,
not the `gpm self-upgrade` the servers use (non-durable in-container).

Refreshed three docs this exposed as stale/incomplete:
- local-setup.md: rewrote the stale "newer Grav RC" section with the durable
  rebuild procedure (recreate gotcha, verify, plugin refresh, non-durability note).
- deploy-cycle.md: Phase 0 now upgrades the local core; state-model notes the
  image as a fourth surface beyond the three server layers.
- stale-grav-version-blocks-api-plugin-install.md: version-authority surfaces
  3 -> 4 (hardcoded Dockerfile URL); clarified .env* GRAV_VERSION governs fresh
  remote installs only, never the local Docker core.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Np4cMQLF77i664CAQXySzU

docs/solutions/integration-issues/stale-grav-version-blocks-api-plugin-install.md

@@ -79,7 +79,7 @@ GPM (Grav Package Manager) only offers a plugin version whose declared Grav requ
 
 ## Prevention
 
-- **Keep `.env.<env>` `GRAV_VERSION` current.** It is the version a *fresh* `make remote-install-<env>` bakes in; a stale value silently installs an old core. After any core upgrade, bump the env file too — self-upgrade only moves the running server. Note this is a *third* version-authority surface alongside `user/config/system.yaml` `gpm.releases` (channel) and `plugins.txt` — they must stay in sync.
+- **Keep `.env.<env>` `GRAV_VERSION` current.** It is the version a *fresh* `make remote-install-<env>` bakes in; a stale value silently installs an old core. After any core upgrade, bump the env file too — self-upgrade only moves the running server. Note this is a *third* version-authority surface alongside `user/config/system.yaml` `gpm.releases` (channel) and `plugins.txt` — they must stay in sync. A **fourth** surface governs the *local Docker* core: the hardcoded `grav-admin-v<ver>.zip` URL in `Dockerfile`. `.env.<env> GRAV_VERSION` governs fresh **remote** installs only — it never touches the local Docker core (which upgrades by an image rebuild, not self-upgrade). See `docs/solutions/tooling-decisions/upgrade-local-grav-core-rebuild-docker-image.md`.
 - **When GPM says "package not found" for a package you know is on your channel, check the target's Grav core version first** (`php bin/grav --version` on the server, or `make remote-diag-<env>`). GPM filters by core compatibility; "not found" often means "no version compatible with your core," not "missing from the index." `gpm index -f` will not help.
 - **Don't trust a top-level install "Success" to mean dependencies installed.** A fresh install can leave a plugin's declared dependency unsatisfied (here `admin2` installed but its `api` dependency didn't). Verify with `ls user/plugins/<dependency>`. The same `ls` guards a *second*, distinct way a plugin ends up non-functional: its **code folder can be missing while its config persists** (e.g. in the per-host env tree), so it looks configured but never loads. Checking `ls user/plugins/<name>` catches both the missing-dependency and the config-without-code cases — see `grav-plugin-config-without-code-wont-enable.md`.
 - **Know the Admin2 ⇄ api coupling.** Admin2 authenticates via the api plugin's `/api/v1` endpoints; a missing or unrouted api plugin makes admin login fail *silently* (login POST 404s, nothing logged). A quick `curl /api/v1/pages` expecting `401` (not `404`) is a good post-deploy smoke check.