feat(browser): add cross-origin iframe support via CDP execution contexts

[xzygis]

Summary

Add cross-origin iframe support using CDP execution contexts, enabling AI agents to discover and interact with elements inside cross-origin iframes without content scripts.

Closes #1077

Problem

Currently, opencli browser state detects iframes but marks cross-origin ones as (cross-origin) with no way to interact. This is because:

1. Snapshot: el.contentDocument returns null for cross-origin iframes (browser security)
2. Execution: Runtime.evaluate via chrome.debugger.sendCommand({ tabId }) only runs in the main frame
3. Protocol: No frameIndex field exists to target a specific frame

Changes

6 files, +153/-4 lines — focused and backward-compatible.

Extension layer

extension/src/protocol.ts

• Add 'frames' to Action union type
• Add frameIndex?: number to Command interface

extension/src/cdp.ts

• Track per-tab frame execution contexts via Runtime.executionContextCreated/Destroyed/Cleared events
• Add registerFrameTracking() for event listener setup
• Add getFrameTree(tabId) — wraps CDP Page.getFrameTree
• Add evaluateInFrame(tabId, expression, frameId) — evaluate JS in a specific frame's context

extension/src/background.ts

• Call registerFrameTracking() during initialization
• Add Page.getFrameTree to CDP_ALLOWLIST
• Add handleFrames() — returns flat list of child frames with index/frameId/url/name
• Modify handleExec() — when frameIndex is set, resolve it to a frameId and use evaluateInFrame()
• Add 'frames' case to command switch

CLI layer

src/browser/daemon-client.ts

• Add 'frames' to DaemonCommand.action union
• Add frameIndex?: number to DaemonCommand

src/browser/page.ts

• Add frames() method — list all child frames
• Add evaluateInFrame(js, frameIndex) — evaluate JS in a specific frame

src/browser/dom-snapshot.ts

• Tag cross-origin iframes with [F0], [F1] indices in snapshot output
• Add hint text: use: opencli browser frames

Design Decisions

1. CDP execution contexts over content scripts: Stays consistent with the existing pure-CDP architecture. No manifest changes needed, no all_frames injection.

2. Frame index (not frameId): Users and AI agents work with simple 0-based indices from opencli browser frames output, not opaque CDP frame IDs.

3. Per-command frameIndex (not stored state): Follows the same pattern as windowFocused and idleTimeout — stateless, per-command relay.

4. Backward compatible: frameIndex is optional everywhere. When absent, all behavior is identical to before.

Usage

# List all frames (including cross-origin)
opencli browser frames

# Output:
# index: 0  url: https://ads.example.com/banner  name: ad-frame
# index: 1  url: https://payment.stripe.com/...   name: payment

# Execute JS in a specific frame
opencli browser eval --frame 1 "document.querySelector('input').value"

# DOM snapshot now shows frame indices
# |iframe|[F0]<iframe src="https://ads.example.com" /> (cross-origin, use: opencli browser frames)

Testing

• ✅ TypeScript type-check passes (0 new errors in CLI, 0 new errors in extension)
• ✅ Backward compatible — no behavioral change without frameIndex