fix: keep install and doctor non-interactive for cookie channels

[0xWinner98]

Summary

Keep install and cookie-backed channel health checks non-interactive by default.

Problem

On macOS, agent-reach install and agent-reach doctor can route into browser-cookie scanning or interactive status commands for cookie-backed channels.

In practice this can trigger repeated Keychain / password prompts when the underlying CLI scans multiple browsers for cookies. It also makes passive health checks hang or retry in places where the user only asked for install or doctor output.

Changes

• install
  • keep automatic browser cookie import disabled by default
  • add explicit opt-in via AGENT_REACH_AUTO_IMPORT_COOKIES=1
  • keep --dry-run messaging aligned with the non-interactive default
• twitter
  • detect saved local credentials from Agent Reach config, ~/.config/xfetch/session.json, or ~/.config/bird/credentials.env
  • skip twitter status / bird check during doctor-style checks
• xiaohongshu
  • detect saved cookies from Agent Reach config or common local cookie files
  • skip xhs status during doctor-style checks
• reddit
  • detect saved credentials from ~/.config/rdt-cli/credential.json
  • skip rdt status --json during doctor-style checks
• docs
  • document the new install behavior
• tests
  • update channel tests to validate passive local-credential detection
  • add CLI coverage for the non-interactive install messaging

Why this shape

install and doctor are passive flows. They work better when they only inspect local state and leave login / browser-cookie import to explicit configuration steps.

That keeps health checks fast, avoids repeated Keychain prompts, and still preserves the existing explicit login/configure workflows.

Validation

• python3 -m py_compile agent_reach/channels/twitter.py agent_reach/channels/xiaohongshu.py agent_reach/channels/reddit.py agent_reach/cli.py
• .venv/bin/pytest tests/test_twitter_channel.py tests/test_channels.py tests/test_cli.py -q
• .venv/bin/pytest tests/test_doctor.py -q
• .venv/bin/pytest -q

Full test run result: 86 passed in 18.83s