acp: config.toml section and --init wizard update

[bug-ops]

Parent: #762

Task

Add ACP configuration section and update the interactive setup wizard.

Acceptance criteria

• [acp] section in config.toml: enabled, agent_name, agent_version, capabilities list
• ZEPH_ACP_* env var overrides
• --init wizard: ACP feature toggle, agent metadata configuration
• Validation in config.validate(): warn if acp enabled but feature not compiled
• Documentation in docs/ for ACP configuration