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/e2e-aggregate-mount.sh

@@ -74,7 +74,7 @@ FIXTURE_TGZ="$(ls "$TMP_DIR"/*.tgz 2>/dev/null | head -1 || true)"
 [ -n "$FIXTURE_TGZ" ] || die "fixture 打包失败"
 
 # ── 引导 scratch profile（web 模板）────────────────────────────────────────
-# 三件套 heredoc 及 pnpm-workspace 的 allowBuilds / minimumReleaseAgeExclude
+# 三件套 heredoc 及 pnpm-workspace 的 minimumReleaseAgeExclude
 # 理由见 e2e-common.sh 的 e2e_write_profile。
 PROFILE_DIR="$DSH_HOME/profiles/web"
 e2e_write_profile "$PROFILE_DIR"