docs: trim CLAUDE.md 305->179 lines; extract code descriptions to docs/

CLAUDE.md is loaded into context on every request, so every line has a
recurring cost. Applies one rule to decide what earns its place: keep what
changes behaviour (rules and gotchas Claude cannot discover before it acts);
extract what merely describes code (Claude reads the code anyway, and prose
about code silently drifts).

The four stale facts fixed in the previous commit were all in the
"describes code" class -- active_trip, the Admin2 version, demo-load's
scope, the gitignore list. None were rules. That is the argument for moving
this material next to what it documents.

Extracted (kept as pointers):
- entry-map + trip-feed-col parameter contracts (56 lines) -> reference/
  architecture.md "Shared partial contracts". CLAUDE.md keeps only the
  invariants: single map path, must assign window.tripMap/homeMap, keep
  trip-feed-col single-purpose, initTripStats depends on MapUtils.
- Prod override runbook (49 -> 9 lines) -> guides/deploy-cycle.md "The env
  override tree", incl. the Twig dev/prod table and WEB_HOST. CLAUDE.md
  keeps the two behavioural rules: never commit prod values, and Admin on
  the server writes to the env tree (so check both config paths, env wins).
- GPX API routes, session auth and the Blob/FormData upload gotcha ->
  guides/gpx-manager.md "How the manager is wired".
- Trip-switch procedure -> guides/trip-switching.md. CLAUDE.md keeps the
  one rule that matters: never re-add pageconfig.parent to post-form.md.
- Also trimmed the dev-command table and custom-plugin table added in the
  previous commit; both largely restated the Makefile and blueprints.

Fixed the guides being pointed into, so the pointers lead to truth:
- trip-switching.md instructed editing a pageconfig.parent that no longer
  exists -- its whole "two files must be updated together" premise was
  obsolete and would have reintroduced the desync it warned about.
- architecture.md: Grav 2.0.4->2.0.7, Admin2 2.0.10->2.0.12, corrected the
  posting pipeline to show cache-on-save injecting parent before the write,
  added entry-actions to the custom-plugin list.
- japan-korea-2026 -> denmark-2026 across guides/reference (docs/solutions
  keeps its historical references intact -- those are incident records).

Verified: every markdown link resolves, every referenced section heading
exists, and each extracted item was confirmed present in its new home.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/guides/gpx-manager.md

@@ -30,7 +30,7 @@ The GPX manager at `/gpx-manager` requires admin login (redirects to login form
 Drop the file directly into the trip folder and push:
 
 ```bash
-cp your-route.gpx /path/to/user/pages/01.trips/japan-korea-2026/
+cp your-route.gpx /path/to/user/pages/01.trips/denmark-2026/
 make content-push
 ```
 
@@ -83,3 +83,25 @@ GPX files are registered as a valid media type in `user/config/media.yaml`, so G
 ```
 
 No manual linking is needed — upload and it appears.
+
+---
+
+## How the manager is wired
+
+| Piece | Detail |
+|---|---|
+| Page | `user/pages/03.gpx-manager/` |
+| Template | `user/themes/intotheeast/templates/gpx-manager.html.twig` |
+| Auth | Login plugin, via `access.admin.login: true` in the page frontmatter — renders the login form when unauthenticated |
+| API | Grav API v1 with **session cookie** auth (`session_enabled: true` in `user/plugins/api/api.yaml`) |
+
+API calls the page makes:
+
+```
+GET    /api/v1/pages{route}/media              # list
+POST   /api/v1/pages{route}/media              # upload (multipart)
+DELETE /api/v1/pages{route}/media/{filename}   # delete
+```
+
+**Upload gotcha:** the selected file is sliced into a plain `Blob` before `FormData.append`, so the third argument is always honoured as the filename. Appending the original `File` lets the browser keep the unslugified name and the slugification is silently ignored.
+