docs(docker): P3 — hoist two-command quickstart, restructure docker/README, add parity CI (#1260)

* docs: hoist Docker zero-install quickstart above npm install path

- Create docs/quickstart-snippet.md as canonical source for the
  two-command flow (curl + docker compose up -d), wrapped in
  <!-- quickstart-snippet-start/end --> markers
- Hoist the snippet into README.md immediately below the deprecation
  banner, above all other install paths
- Rename old npm-only "## Quick Start" to "## Install on Host (npm)"
  and move it below the Docker quickstart

* docs(docker): restructure README with zero-install first and migration section

- Reorder top-level sections: zero-install (canonical snippet with
  markers), choosing an image, power-user ccs docker, prebuilt image,
  connect your app to CLIProxy, migration, env vars, troubleshooting
- Add deprecation banner at the top pointing at the migration section
- Add ## Migration from ccs-dashboard:latest section with step-by-step
  instructions covering compose down, data preservation, named volume
  vs bind-mount path, and compose up with the new image
- Keep P1's Choosing an image table and P5's Connect Your App to
  CLIProxy section intact, just repositioned

* test(docs): parity check for quickstart snippet across README files

Assert README.md and docker/README.md both contain the canonical
quickstart block verbatim, anchored by marker comments. Exits non-zero
and prints a diff on any drift.

* ci(docs): wire quickstart-parity test on push and PR

Runs tests/docs/quickstart-parity.sh on self-hosted runner whenever
docs/quickstart-snippet.md, README.md, docker/README.md, or the
test/workflow files themselves change. Fails fast on snippet drift.

tests/docs/quickstart-parity.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# quickstart-parity.sh
+# Assert that README.md and docker/README.md both contain the canonical quickstart
+# snippet verbatim (anchored by marker comments).
+# Usage: bash tests/docs/quickstart-parity.sh  (from repo root)
+set -euo pipefail
+
+SNIPPET=$(awk '/<!-- quickstart-snippet-start -->/,/<!-- quickstart-snippet-end -->/' docs/quickstart-snippet.md)
+
+fail=0
+for f in README.md docker/README.md; do
+  file_block=$(awk '/<!-- quickstart-snippet-start -->/,/<!-- quickstart-snippet-end -->/' "$f")
+  if ! diff -q <(printf '%s' "$SNIPPET") <(printf '%s' "$file_block") >/dev/null 2>&1; then
+    echo "[X] $f quickstart snippet drift detected" >&2
+    echo "--- canonical (docs/quickstart-snippet.md) ---" >&2
+    printf '%s\n' "$SNIPPET" >&2
+    echo "--- found in $f ---" >&2
+    printf '%s\n' "$file_block" >&2
+    fail=1
+  else
+    echo "[OK] $f snippet matches canonical"
+  fi
+done
+
+exit "$fail"