refactor(policy): separate launcher and subprocess capabilities

[tumberger]

Summary

• keep foreground terminal compatibility independent from process-creation authority
• require callers and the CLI to opt into subprocess support explicitly
• preserve the current CLI launch contract while keeping current-process policies minimal

Stack

1. This PR — separate launcher and subprocess capabilities
2. complete CLI policy files
3. replace the legacy authoring subsystem with CLI-local agent presets

Verification

• make check
• strict macOS and Linux-target Clippy
• workspace tests

Draft until the complete stack is reviewed together.