mecatui: prioritize and expand dynamic card help

Problem

Dynamic card help/footer text currently has no semantic priority model. A rebound key label can make a one-line hint exceed its available width. The short-term sentinel renderDynamicCardChromeLine truncates one Subagent-roster footer with ...; it deliberately does not solve help discoverability.

Follow-up

• Represent card-help hints as ordered semantic items with explicit priority.
• Pack items into the available display-cell width, dropping lower-priority items rather than truncating an opaque joined string.
• Provide a context-scoped ? help modal that lists the complete live/rebound key bindings and can scroll.
• Define narrow-terminal fallbacks and preserve terminal sanitization/display-width correctness.
• Audit and migrate every renderDynamicCardChromeLine sentinel call site deliberately; do not apply it to prose/content rows.

Context

This follows the card-layout work in PR #1105, which fixes padding-before-wrap and introduces the single-line sentinel for the Subagent roster footer.