feat: image-baked agent skills + pi-devbox-environment skill (v1.2.0)

Ship skills inside the image (independent of any mounted skillset repo):
- rootfs/usr/local/share/pi-devbox/skills/<name>/ symlinked into
  ~/.agents/skills/ by entrypoint-user.sh (foreign-link, survives volume
  recreate, never clobbers a skillset/user skill of the same name).
- New pi-devbox-environment skill: persistence model, host/LAN SSH
  reachability, split-DNS mechanisms, interactive-vs-tool-shell alias
  gotcha, tmux 0-index, uv-first Python, pi-studio reachability. Agnostic
  to host OS / hostnames / domains / nameservers (discovered at runtime).
- Dockerfile.variant appends pi-global-AGENTS.append.md onto pi-toolkit's
  pi-global-AGENTS.md (single global slot) so the skill is loaded
  proactively; gated on /usr/local/lib/pi-devbox/. Idempotent.
- smoke-test: baked-skill + append-snippet + merged-marker presence and a
  runtime symlink assertion.
- docs: README 'Agent skills' section, AGENTS.md layout, DOCKER_HUB.md;
  moved studio-tex roadmap to v1.3.0.

pi 0.79.7 -> 0.79.10 (auto-resolved from npm latest at build).

Dockerfile.variant

@@ -94,6 +94,24 @@ RUN set -e && \
     echo "pi-fork at $(cd /opt/pi-fork && git rev-parse --short HEAD)" && \
     echo "pi-observational-memory at $(cd /opt/pi-observational-memory && git rev-parse --short HEAD)"
 
+# ── pi-devbox awareness: append our pointer to the global AGENTS.md ──
+# pi loads a SINGLE global instruction file (~/.pi/agent/AGENTS.md), which
+# pi-toolkit's install.sh re-symlinks to /opt/pi-toolkit/pi-global-AGENTS.md on
+# every container start. There is no second global slot, and that file is
+# root-owned (not writable by the runtime user), so we compose at BUILD time:
+# append the pi-devbox managed block to pi-toolkit's file here, after the clone.
+# Idempotent via a marker grep so a rebuilt layer never double-appends. This
+# makes every container proactively aware of the pi-devbox-environment skill;
+# the snippet itself is gated (only fires when /usr/local/lib/pi-devbox exists).
+RUN if [ -f /opt/pi-toolkit/pi-global-AGENTS.md ] && \
+       ! grep -q 'pi-devbox:managed-block' /opt/pi-toolkit/pi-global-AGENTS.md; then \
+      printf '\n' >> /opt/pi-toolkit/pi-global-AGENTS.md && \
+      cat /usr/local/share/pi-devbox/pi-global-AGENTS.append.md >> /opt/pi-toolkit/pi-global-AGENTS.md && \
+      echo "appended pi-devbox block to pi-global-AGENTS.md" ; \
+    else \
+      echo "pi-devbox block already present or pi-global-AGENTS.md missing (skipped)" ; \
+    fi
+
 # ── Optional: pi-studio (:latest-studio variant) ─────────────────────
 # pi-studio (omaclaren/pi-studio) is a pi-package + theme providing a
 # two-pane browser workspace: prompt/response editor, KaTeX/Mermaid live