feat(tui): render policy proposal inbox metadata

[zredlined]

Description

Update the TUI draft policy panel into the MVP inbox experience for single-sandbox review.

Context

Parent: #1062
RFC artifact: https://github.com/NVIDIA/OpenShell/blob/feat/agent-driven-policy-management/rfc/0001-agent-driven-policy-management.md

This is part of the locked Agent-Driven Policy Management MVP. GitHub issues are the development source of truth; Linear is only a roadmap pointer.

The MVP keeps polling rather than adding server-streaming push.

Definition of Done

• TUI renders human_summary as the primary proposal headline when present.
• TUI shows intent_summary as agent-supplied context.
• TUI shows validation_result as a badge/status.
• TUI shows an explicit L4/no-method-path scoping warning when applicable.
• Rejected chunks show persisted guidance.
• Poll cadence is tightened to roughly 2 seconds for the active sandbox view.
• Existing approve/reject hotkeys continue to work.