docs: record the 0.1.6-alpha.2 adaptation and resync the user docs

Adds the per-version adaptation record (docs/plans/2026-09-21-…-adaptation.md),
rewrites AGENTS.md §3 for the new baseline (the two silent contract breaks, the
terminal/browser handoff, the new official plugin-manager surface, the three
ui-primitives breaks, the widened primitives bare-import set), and updates both
READMEs and the integration guide for the v0.20.0 feature set.

Also retires the support text the terminal deletion made false: the install
flow is one step again (no build-script approval), the FAQ's two node-pty rows
collapse into one pointing at DSH's own terminal, the platform note no longer
promises prebuilt native binaries (there are no native deps left), and the
installers drop the -Repair mode and their allowBuilds writes along with
pnpm-workspace.yaml's now-empty allowBuilds block. Removes the dead
browserNoSandboxWarning locale key (its setting went with the plugin's browser)
and tightens theme.spec's capsule guard so a literal colour fails it.

The e2e resolver now refuses a PATH  that is not on the baseline line:
this machine's PATH  is 0.1.6-alpha.1, and running the mount lane against
it fails at 'the bundle was not registered' with a zero exit from the CLI —
a misleading dead end worth one explicit error instead.

scripts/package-registry.mjs

@@ -10,7 +10,7 @@
  *
  * The registry install copies the whole source directory (`cp -r`), so
  * installing the repo root would drag in `node_modules/` (machine-local
- * `link:` devDependency symlinks, node-pty binaries) and `.git/`. This
+ * `link:` devDependency symlinks) and `.git/`. This
  * staging directory carries exactly the files the manifest references plus
  * the docs — same layout as the dsh-subagent-tree `registry/` pattern. It is
  * gitignored and rebuilt from scratch on every run.