Replace the typewriter-brand + random-hash naming scheme for
auto-generated git worktree branches with an adjective-noun approach
(e.g. `swift-falcon`, `calm-meadow`).

- Removed the ~650-entry typewriter brand name list and hash generation
- Added 235 adjectives and 228 nouns (~53,500 unique combinations)
- Branch names are now checked for exact-match collisions against
existing branches
- Updated error messaging

Closes AI-101

Release Notes:

- Improved auto-generated git worktree branch names to use friendlier
adjective-noun pairs (e.g. `swift-falcon`) instead of typewriter brand
names with random hashes.