feat: add local platform capability list command

[can4hou6joeng4]

What

• Add boss platforms as a local-only platform capability matrix command.
• Include candidate/recruiter registration status, default platform, aliases, and capability groups for readonly/write/local features.
• Explicitly mark qiancheng / 51job as placeholder-only with not_supported real platform capabilities.
• Register the command in schema and document the no-network usage in both READMEs.

Why

• Gives agents and users a stable way to inspect platform availability before selecting a platform.
• Keeps 51job expectations explicit without touching login, browser, CDP, or real network workflows.

Testing

• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python -m pytest tests/ -q → 1433 passed
• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python -m ruff check src/boss_agent_cli/commands/platforms.py tests/test_platforms_cmd.py
• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python -m mypy src/boss_agent_cli/commands/platforms.py
• PATH="$PWD/.venv/bin:$PATH" boss --json platforms | python -m json.tool