fix: add terminfo capability guard to interactive.d scripts

[alohays]

Summary

• Add dotfiles_term_capable() helper to lib.sh that validates both TERM value and actual terminfo availability via tput
• Replace static case TERM in ''|dumb) guards in 6 interactive.d scripts with the new helper
• Guard p10k loading in 80-prompt.sh so fallback prompt is used when terminfo is unavailable

Problem

When SSH-ing from Ghostty terminal (TERM=xterm-ghostty) to a host without the xterm-ghostty terminfo entry, ZLE plugins (p10k, autosuggestions, fast-syntax-highlighting) compute incorrect cursor positions because tput fails and $COLUMNS=0. This causes progressive character garbling (e.g., ls → lsls, claude → claudecllaauudclaude).

Root Cause

The existing case TERM in ''|dumb) guards only check for empty/dumb TERM values. xterm-ghostty passes this guard (non-empty, non-dumb) but has no terminfo entry on the remote, causing all terminal capability queries to fail silently.

Fix

dotfiles_term_capable() checks both conditions:

1. TERM is non-empty and not dumb
2. tput cols actually succeeds (terminfo is usable)

Files Changed

• modules/core/home/.config/dotfiles/lib.sh — new dotfiles_term_capable() helper
• modules/prompt/home/.config/dotfiles/interactive.d/00-p10k-instant-prompt.zsh — add guard (had none)
• modules/core/home/.config/dotfiles/interactive.d/70-zsh-plugins.sh — upgrade guard
• modules/prompt/home/.config/dotfiles/interactive.d/80-prompt.sh — guard p10k, keep fallback
• modules/prompt/home/.config/dotfiles/interactive.d/81-completion.zsh — upgrade guard
• modules/prompt/home/.config/dotfiles/interactive.d/82-rich-plugins.zsh — upgrade guard
• modules/prompt/home/.config/dotfiles/interactive.d/83-rich-fzf.sh — upgrade guard

Test plan

• SSH into host with xterm-ghostty terminfo installed → full functionality (p10k, autosuggestions, syntax highlighting)
  • Verified: ssh mac-mini — no character garbling, p10k prompt renders correctly, autosuggestions and syntax highlighting work
  • Automated: TERM=xterm-ghostty tput cols → 80, infocmp xterm-ghostty → OK
• SSH into host without terminfo → graceful fallback (simple prompt, no garbling)
  • Verified: TERM=xterm-nonexistent zsh -c 'source ~/.zshrc; dotfiles_term_capable && echo pass || echo blocked' → blocked
  • Plugins correctly skipped, no ZLE cursor errors
• Simulate: TERM=xterm-nonexistent zsh -l → plugins skipped, fallback prompt works
  • Verified: dotfiles_term_capable returns non-zero, all interactive.d guards fire, fallback prompt displayed
• Local shell (non-SSH) → no regression, everything works as before
  • Verified: local dotfiles_term_capable → pass, all plugins load normally

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