Fix README inaccuracies: install instructions, example output, worktree paths

[that-github-user]

Summary

Several inaccuracies in README found during audit:

1. Install instructions — npm install -g thinktank won't work because dist/ isn't built during install (see Add npm publish workflow and prepublish build step #28)
2. Next steps section — shows git diff --no-index /dev/null <worktree>/... which is wrong. Should be cd <worktree> && git diff HEAD
3. Example output — shows worktree paths as generic placeholders. Should show actual paths or note they're in system temp directory
4. Model flag — --model opus shown but no docs on valid model names

Acceptance criteria

• Install instructions accurate (or marked as "from source" until npm publish works)
• Worktree inspection instructions are correct
• Model names documented
• Example output matches actual CLI output format

[that-github-user]

Fixed in #44. Replaced npm install -g thinktank (not yet published) with clone + build + link instructions. Added Models section documenting --model flag options (sonnet, opus, haiku, full claude-* IDs).