feat: add "Devin for Terminal" as a supported agent in skills CLI

[minatoaquaMK2]

Register "Devin for Terminal" (https://cli.devin.ai/docs) as the 42nd supported non-universal agent in the skills CLI, enabling skills add <repo> -a devin and all related commands (list, remove, sync, etc.).

Agent configuration:

• AgentType union: added 'devin' literal to src/types.ts
• Agent config: name 'devin', displayName 'Devin for Terminal', skillsDir .devin/skills (project-level), globalSkillsDir ~/.config/devin/skills (XDG-style global), detectInstalled via existsSync(~/.config/devin)
• Follows the goose pattern (non-universal agent + XDG config paths)

Skill discovery:

• Added .devin/skills to prioritySearchDirs in src/skills.ts so discoverSkills() finds skills placed in .devin/skills/<name>/

Tests added (39 new tests, 441 total passing):

• tests/devin-agent.test.ts: 17 unit tests covering config fields, detection logic (both branches), non-universal classification, and uniqueness guards (no duplicate names/dirs)
• tests/devin-discovery.test.ts: 5 tests for skill discovery in .devin/skills/, cross-agent discovery, and deduplication
• tests/devin-installer.test.ts: 17 integration tests for symlink install, copy install, global path resolution, agent recognition, listInstalledSkills attribution, and end-to-end consistency

Documentation & metadata:

• README.md: agent table updated (42 agents), discovery paths include .devin/skills/, generated via bun scripts/sync-agents.ts
• package.json: 'devin' keyword added via sync script
• bun scripts/validate-agents.ts: passes ("All agents valid.")

Verification: 441 tests pass, 6 pre-existing type errors (no new), Prettier formatting clean, validate-agents.ts passes.