feat: add AGENT_BROWSER_CDP env var for --cdp flag

[bukacdan]

Problem

When agent-browser is used programmatically by an AI agent (e.g., inside an agentic loop that drives a
browser), connecting to an existing browser via CDP currently requires one of two approaches:

1. Pass --cdp on every command -- the agent must remember to include --cdp 9222 (or a WebSocket URL)
   on every single agent-browser invocation throughout its session. In practice, agents can and do forget
   flags between calls, leading to commands silently launching a new browser instead of controlling the intended
   one.

2. Run agent-browser connect at the start -- this persists the CDP endpoint in the session, but it
   relies on the agent executing the right setup command first and maintaining the same session for every
   subsequent call. If the session is lost or the connect step is skipped, the same silent-failure mode occurs.

Both approaches are fragile in agentic contexts because they depend on the agent's behavioral consistency
across many invocations -- something that is inherently unreliable.

Solution

Add AGENT_BROWSER_CDP environment variable support, following the established AGENT_BROWSER_* convention
already used by --proxy (AGENT_BROWSER_PROXY), --provider (AGENT_BROWSER_PROVIDER), --auto-connect
(AGENT_BROWSER_AUTO_CONNECT), and others.

When AGENT_BROWSER_CDP is set, every agent-browser command automatically uses the specified CDP endpoint
without requiring any flags. The CLI --cdp flag still takes precedence when both are present, preserving
full backward compatibility.

# Set once in the agent's environment
export AGENT_BROWSER_CDP=9222

# Every command connects to the right browser, no flags needed
agent-browser snapshot
agent-browser click @e1
agent-browser fill @e2 "hello"

This makes CDP connections deterministic and reliable regardless of the agent's behavior -- the environment
is set by the orchestrator, and individual commands cannot "forget" it.

Changes

Functional (1 line):
- cli/src/flags.rs -- read AGENT_BROWSER_CDP env var as fallback for the cdp field, identical to the pattern
used by every other env-backed flag

Documentation (4 files):
- cli/src/output.rs -- --help text and environment variable listing
- README.md -- options table and CDP Mode section
- skills/agent-browser/SKILL.md -- AI agent skill reference
- docs/src/app/cdp-mode/page.mdx -- docs site CDP Mode page

Compatibility

- Fully backward compatible; no existing behavior changes
- --cdp flag overrides the env var (CLI args are parsed after env var defaults, matching the existing
pattern)
- Existing conflict checks (--cdp + --extension, --cdp + --auto-connect) work unchanged since they test
flags.cdp.is_some() regardless of source
- All 162 existing tests pass

[vercel]

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