feat(#71): add .agent-browserrc.json config file support

[giulio-leone]

Summary

Adds support for .agent-browserrc.json config files for persistent project-level and user-level settings.

Config Loading Order (lowest to highest priority)

1. ~/.agent-browserrc.json (user-level)
2. .agent-browserrc.json in CWD (project-level, overrides user)
3. Environment variables (override config file)
4. CLI flags (override everything)

Example Config

{
  "headed": true,
  "browser": "firefox",
  "profile": "/path/to/profile",
  "stealth": true,
  "proxy": "http://proxy:8080",
  "extensions": ["/path/to/ext1"],
  "allowFileAccess": true
}

Changes

• cli/src/flags.rs: Add load_config() function that reads and merges config files, use as lowest-priority defaults in parse_flags()
• No new dependencies (serde_json already in Cargo.toml)
• Silent fallback on missing/malformed config files

Closes #71

[vercel]

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