feat(conductors): add conductor sessions

[doyled-it]

Summary

• Adds conductor and child session roles, nested session-list navigation, and Sub-session Details alongside the existing Preview and Details panes.
• Adds conductor session creation, mediated action storage/policy/CLI execution, and an autonomous heartbeat worker with action limits and retry hardening.
• Adds conductor workspace attach with a read-only sidecar panel, raw attach fallback, and README documentation.
• Cleans up detached orphan agent-view tmux sessions that no stored session references before startup monitors create more tmux sessions.

Test Plan

• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo build --release --all-features
• prek run --all-files
• Targeted orphan tmux cleanup and tmux session parser tests
• Final branch review completed with no remaining findings

Closes #35