fix(setup): make Ghostty install bulletproof

[bogdanmatasaru]

Problem

On machines that already had a manually installed /Applications/Ghostty.app (direct .dmg, corporate image, MDM), brew install --cask ghostty fails with Error: It seems there is already an App at '/Applications/Ghostty.app'. The script warned and continued, and validate() then reported Ghostty (cask) — MISSING even though Ghostty was present — producing the alarming "1 problem(s)" output a colleague hit.

Fix

• install_ghostty — sanity-checked, layered install:
  1. skip if Homebrew already manages the cask (idempotent);
  2. adopt a pre-existing Ghostty.app into Homebrew (--adopt) instead of colliding with it;
  3. retry once after brew update for flaky networks;
  4. --force replace as a last resort (only when an app is actually present).
• Verify the outcome after every attempt (brew list --cask ghostty) instead of trusting the exit code.
• validate() now detects Ghostty by brew cask or app bundle, removing the false-positive MISSING for non-brew installs.

Validation

• bash -n clean; shellcheck clean for the new code.
• Dry-run renders a single clean preview line (no spurious retry/force noise).
• --check passes via the new ghostty_present predicate.
• New code is comment-free, self-documenting, and DRY (shared presence predicates reused by install + validate).

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