v1.8.6: adopt pi 0.84.3 + mempalace 3.8.0, close the v1.8.5 doc/observability gaps

Three coupled pieces of work, all of which ride on the base rebuild that the
mempalace bump forces anyway.

DRIFT ADOPTED
- pi 0.84.2 -> 0.84.3. Its release notes carry a "Breaking Changes" line
  (GoogleThinkingLevel -> GoogleApiThinkingLevel). Audited before adopting:
  zero references across all four vendored companions (pi-fork,
  pi-observational-memory, pi-atelier, pi-studio), so it is inert for us. The
  reason to adopt is two skill-discovery fixes that land directly on v1.8.5's
  vendored-skill work: nested Markdown skills inside grouping directories were
  not discovered, and root README.md/AGENTS.md in skill dirs were reported as
  broken skills.
- mempalace core 3.7.1 -> 3.8.0. Additive/reliability only. Its sync fix
  (#2320/#2322) stops sync --apply deleting drawers whose source_file was
  unreachable *at that moment* -- which does NOT relax the standing landmine
  against sync on the shared palace, because that landmine is about paths
  permanently absent from whichever host runs the sync. Different failure
  shape; the caution stands.

DOCS -- three defects, one of them public
- DOCKER_HUB.md advertised "neovim (LazyVim defaults)". Nothing in the image
  installs LazyVim; the only nvim config is a 19-line sysinit.vim. CI PATCHes
  this file into the Docker Hub description on every release, so this was a
  false claim published to the world. Removed.
- agent-browser + Playwright + Chromium is the single largest addition in the
  image (~625 MB) and had zero mentions in README, DOCKER_HUB or THIRD_PARTY --
  it was documented only to agents, in the AGENTS.md managed block. Now
  documented to humans, including the Chromium licence dimension.
- typst and socat appeared in README prose but not in the "What's inside"
  inventory. Added.

OBSERVABILITY -- the three gaps v1.8.5 listed as still open
- build-manifest.json now records mempalace core, read from the live binary
  (ground truth, not the build ARG). Placed as a sibling of pi_version rather
  than inside components{}, because pi-devbox-version renders that map through
  [0:12] and would truncate a version string.
- smoke asserts the pi-observational-memory clone actually CONTAINS the ce9fc98
  auth fix, pinned to src/runtime.ts. Deliberately not a repo-wide grep: two of
  the three markers also live under tests/, so the repo-wide form stays green
  with the fix site reverted. That is the third false-green of this exact family
  in this repo (canary phrase in both snapshots; reconciler fixture using a
  non-owned name; now this) -- pin containment checks to the fix site.
- smoke asserts the feeder's pi@<device> agent default behaviourally. The
  earlier audit concluded this needed a --print-config added upstream; it does
  not. AGENT is assigned before arg parsing, so `bash -x mempalace-pi-session
  --help` observes the real resolution with no toolkit change. Two-sided:
  device set => pi@<device>, unset => must not be pi@*.
- pi-devbox-version now prints a palace: line with the same live-vs-baked drift
  detection pi already had. This matters more than it looks: mempalace is the
  one component that is both client (here) and server (synlig), so skew between
  them is a real failure mode. Degrades quietly on pre-v1.8.6 images.

Deferred deliberately: a native arm64 act_runner on tor-ms22 (the current
runner is on synlig, x86_64, so every arm64 layer ships QEMU-emulated).
Analysis and caveats filed to the palace rather than actioned here.

DOCKER_HUB.md

@@ -65,12 +65,19 @@ The entrypoint deploys/registers all of these on first container start. Re-runni
 ### Document and image tooling
 
 - **pandoc** — universal Markdown↔HTML/Org/RST/etc. conversion. Useful well beyond pi: agent-driven doc exports, format conversion, etc.
+- **Typst** — markup-based typesetting, used as pandoc's `--pdf-engine`
 - **graphviz** (`dot`) — diagram rendering pipelines
 - **imagemagick** (`magick`) — image conversion / resizing
 
+### Browser automation
+
+- **agent-browser** — CLI for driving a real browser (open pages, click/fill/`eval`, snapshot the DOM, screenshots) so agents can verify front-end work instead of guessing
+- **Playwright** + a headless **Chromium** are pre-installed and pinned together; `AGENT_BROWSER_EXECUTABLE_PATH` is preset to the baked browser, so `agent-browser open <url>` works out of the box with no setup
+- **socat** — TCP bridge used to expose the pi-studio server outside the container's loopback
+
 ### Modern CLI tooling
 
-- **Editor**: neovim (LazyVim defaults), tmux (configured for 0-indexed sessions)
+- **Editor**: neovim (system-wide `termguicolors` default; bring your own config/plugins), tmux (configured for 0-indexed sessions)
 - **Search/nav**: ripgrep, fd, fzf, zoxide
 - **Display**: bat, eza, htop, tree
 - **Data**: jq, yq