Add help overlay explaining hook status indicators and keybindings

[glnds]

Context

The status bar now shows several hook-related indicators that need explanation:

Hook status labels

• ✓hook — cibars signal hook installed in local .git/hooks/pre-push
• ✓g-hook — installed in global hooks dir (core.hooksPath)
• ✓gd-hook — installed in global hooks dir with delegation to local hooks
• ⚠hook:override h=fix — hook exists in .git/hooks/ but is shadowed by core.hooksPath
• h=install pre-push hook — no hook installed yet

What to document

• Explain each hook status label
• Explain what h does in each state (install, fix shadowing)
• Explain delegation: global hook calls local .git/hooks/pre-push if present
• Explain core.hooksPath interaction
• Document all keybindings: b=boost, h=hook, l=relink, e=expand, q=quit
• Consider a ? or F1 key to show/hide a help overlay

Implementation ideas

• Help overlay rendered as a centered modal over the TUI
• Toggle with ? key
• Lists all keybindings + explains current hook status