Rewrite DOCKER_HUB.md as a hand-maintained slim template

The previous derive-from-README mechanism (split_sections, SECTION_RULES,
TRIM_SUBSECTIONS, REPLACEMENTS) generated a 24 997 byte Hub doc with
3 byte headroom against the 25 kB Hub limit. Every README addition
forced a 'trim something else first' exercise, and the resulting copy
was awkward (terse, repetitive linkbacks injected mid-section).

Replace with a single hand-maintained HUB_TEMPLATE constant. The Hub
doc is now intentionally slim (~5.5 kB, ~78 percent headroom) and
focuses on what Hub readers actually need: elevator pitch, image
variants, quick start, what's inside, auth, persistence, and link-outs
to the gitea README for depth.

Trade-off: when image-variants or quick-start change, update
HUB_TEMPLATE here too. That coupling is now explicit and local rather
than spread across SECTION_RULES + REPLACEMENTS + TRIM machinery,
and most README edits no longer require regenerating DOCKER_HUB.md
at all.

Generator simplified from 323 lines to 199 lines (270-line net
reduction across the script + DOCKER_HUB.md). README and Hub doc are
now independent surfaces.

CHANGELOG and AGENTS updated to reflect the new coupling. Release-day
checklist tightened: README -> regenerate DOCKER_HUB ONLY if
HUB_TEMPLATE changed -> promote CHANGELOG -> grep AGENTS -> commit
-> tag.

CHANGELOG.md

@@ -19,6 +19,11 @@ Image changes (will ship in the next tagged release):
 
 - **Fix:** `pi install npm:<pkg>` (and any `npm install -g`) by the `developer` user no longer EACCES against the system npm prefix. `NPM_CONFIG_PREFIX` is now `/home/developer/.pi/npm-global` and the prefix's `bin/` is prepended to `PATH`. The directory lives on the `devbox-pi-config` named volume, so user-installed pi packages (themes, skills, extensions) survive container recreation and image rebuilds. Build-time `npm install -g` calls (opencode, pi, oh-my-opencode-slim) are unaffected because the new ENVs are declared after those steps in the Dockerfile, so the baked binaries still install to `/usr` and are not shadowed by the volume mount.
 
+Docs-only (the DOCKER_HUB.md change can be patched live to Hub without a CI rebuild; AGENTS.md change is repo-internal):
+
+- **Hub doc rewrite:** `DOCKER_HUB.md` is now generated from a hand-maintained `HUB_TEMPLATE` constant in `scripts/generate-dockerhub-md.py` instead of a section-by-section transformation of `README.md`. Drops from 24 997 bytes (3 byte headroom) to 5 551 bytes (~78% headroom). The old derive-from-README mechanism (`SECTION_RULES`, `TRIM_SUBSECTIONS`, `REPLACEMENTS`, `split_sections`, `trim_subsections`) is gone — README and Hub doc are now independent surfaces. Hub copy stays slim and links out to the gitea README for full depth (build args, multi-user setup, AWS Bedrock walkthrough, MemPalace deep-dive, language-specific dev sections). Trade-off: image-variants table and quick-start flow are now coupled to `HUB_TEMPLATE` and need a manual edit when they change — explicit and local rather than spread across rules.
+- **AGENTS.md:** "Documentation coupling on release" rule updated — README edits no longer require regenerating DOCKER_HUB.md. Release-day checklist tightened.
+
 ## v1.14.41b — 2026-05-08
 
 **Optional pi as second harness.**