feat: add WebSocket message inspection tools

[johan-gorter]

Summary

Adds three read-only tools that let an agent inspect the WebSocket traffic of
the selected page — the one common network primitive the server currently
can't see, since Puppeteer does not expose WebSocket frames:

• list_websocket_connections — connections of the selected page, with a
  stable wsId, status, and sent/received counts.
• list_websocket_messages — messages of a connection (oldest first) as
  single-line previews, filterable by direction and by a payload substring,
  with pagination.
• get_websocket_message — the full payload of one message, optionally saved
  to a file inside the configured workspace roots.

How it works

Because Puppeteer doesn't surface WebSocket frames, a WebSocketCollector
consumes the CDP Network.webSocket* events directly from the page session
that is already enabled. It follows the existing PageCollector pattern used
by the console and network collectors, so it plugs into the same
per-page/per-navigation storage, disposal, and stable-id machinery with no
base-class changes. The CDP session is obtained via page._client(), the same
way PageCollector and WaitForHelper already do.

Capture starts the moment the page is inspected — reload to capture a
connection from its start. This is documented in the tool descriptions, the
README, and the skill.

Bounded memory

Retention is capped per connection so long-lived sockets can't grow unbounded:
the last 500 data messages within a 2 MB total budget, each stored payload
capped at 100k characters. Dropped and truncated messages are counted and
marked explicitly rather than silently discarded. Control frames (ping/pong)
are not recorded. All constants are exported and easy to tune — happy to adjust
if you'd prefer different defaults.

Single-page-app correctness

Field testing against a real SPA surfaced a subtlety worth calling out: an app
that rewrites its URL with history.pushState triggers Puppeteer's
framenavigated, but that is a same-document navigation and does not close
WebSockets. The collector therefore:

• decides a connection is dead on Runtime.executionContextsCleared
  (cross-document navigations only), not on framenavigated, and
• carries still-open connections across each navigation split, so a socket
  opened during page boot stays visible.

list_websocket_connections also accepts includePreservedConnections to list
connections closed by earlier navigations.

Docs & tests

• README, tool reference (npm run gen), CLI options, and the
  chrome-devtools skill are updated.
• 13 tests: unit tests for retention/truncation/control-frame handling, plus
  integration tests against a new dependency-free WebSocket echo endpoint in
  the test server (capture, direction/text filtering, truncation, same-document
  navigation survival, preserved connections, drop accounting, and the
  unknown-connection error path).

Open questions for reviewers

• Retention defaults (500 / 2 MB / 100k) — reasonable, or would you prefer
  different limits or configurability?
• includePreservedConnections and filePath saving add surface area; happy
  to drop either for a leaner first version if you'd rather.

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