feat: replace playwright area snapshot with cdp ax tree

[cryptotavares]

What

Replaces Playwright's ariaSnapshot() with raw CDP accessibility APIs for building the accessibility tree that agents consume, and extracts a shared CDP session lifecycle helper.

Why

The discovery layer relied on Playwright's YAML-based ariaSnapshot() API, which has no concept of backend DOM node IDs. Generic-role elements (popover menu items, custom list rows, unnamed wrappers) could never be mapped to stable Playwright selectors — they were silently dropped from snapshots, leaving agents blind to actionable UI that doesn't use standard ARIA roles.

How

CDP accessibility integration — collectTrimmedA11ySnapshot now opens a CDP session and queries the browser's accessibility tree directly:

Scope	CDP method	Fallback
Full page	Accessibility.getFullAXTree	—
Scoped (root selector)	Accessibility.queryAXTree	Accessibility.getPartialAXTree

Every AX node now carries its backendDOMNodeId, enabling DOM-level selector resolution for nodes that don't map to a clean ARIA role selector.

Selector resolution strategy — Nodes are resolved to Playwright selectors via a tiered approach:

1. Included ARIA roles (button, link, heading, etc.) → cheap role=…[name="…"] selector, no CDP round-trip
2. Generic/non-standard roles → DOM.resolveNode → Runtime.callFunctionOn to synthesize the most stable CSS selector available:
   • Unique data-testid → [data-testid="…"]
   • Unique id → #…
   • Ancestor path → div > div:nth-of-type(1) > span
   • Text fallback → text="…" (last resort, with debug warning)

Named generic nodes — Generic-role nodes that carry a non-empty accessible name (e.g. popover action items) are now included in snapshots. Unnamed generic wrappers are still excluded but their children are traversed.

CDP session lifecycle — A new withCdpSession(page, fn) helper centralises session creation → use → guaranteed detach, shared with the clipboard tool.

Test coverage — Comprehensive tests for CDP session lifecycle (detach on success/failure), scoped snapshots (queryAXTree and getPartialAXTree fallback), generic node inclusion/exclusion, DOM selector resolution, ignored node handling, and ancestor-path selectors. Coverage thresholds updated accordingly.