evolve plan mode: investigate-first interview, critic review, calmer ui

[jbj338033]

Summary

Evolves plan mode along the design researched in docs/research/ — synthesizing deep-interview, ralplan, brainstorming, gstack, and ouroboros into one workflow that fits goat's full-screen TUI and human approval gate. The agent investigates before asking, routes each need (descriptive facts -> investigate, costly/irreversible decisions -> ask, trivial reversible ones -> fold into the plan), interviews one pivotal question at a time with a one-line reason and a recommendation, writes a scannable woven plan with only the sections the work needs, and can delegate read-only critic agents that only flag findings (never silently edit the plan). Also adds a deterministic placeholder gate before proposing, makes the critic agent real (it was referenced by the prompt but did not exist), and gives plan mode a calmer signal: a composer plan label + color and a lighter approval overlay.

Verification

• cargo fmt --all — passed
• cargo clippy --workspace --all-targets -- -D warnings — passed
• cargo test --workspace — passed