feat(integrations): add Devin for Terminal skills-based integration

[ivishalgandhi]

Closes #2346

Summary

Adds Devin for Terminal as a skills-based integration. Users can now run specify init --ai devin (or --integration devin) to scaffold a project with Devin-compatible skills at .devin/skills/speckit-<command>/SKILL.md and an AGENTS.md context file at the repo root.

Changes

• New DevinIntegration class in src/specify_cli/integrations/devin/ (subclasses SkillsIntegration, ~30 lines)
• Registered in INTEGRATION_REGISTRY (alphabetical, between cursor-agent and forge)
• Added catalog entry, docs row in docs/reference/integrations.md, and listing in .github/ISSUE_TEMPLATE/agent_request.yml
• Updated init "Next Steps" panel to show /speckit-<command> (hyphen) syntax for Devin, matching how Devin invokes skills by directory name (consistent with Claude/Cursor/Copilot in skills mode)

Testing

Unit tests — 27/27 pass

Inherits the shared SkillsIntegrationTests mixin (registration, config, file creation, manifest tracking, install/uninstall roundtrip, frontmatter validation, etc.).

$ uv run pytest tests/integrations/test_integration_devin.py -v ============== 27 passed in 0.41s ==============

Full suite — no regressions

Manual end-to-end with actual Devin CLI

Installed Devin for Terminal locally and ran:

```
❭ /speckit-constitution python project no hardcoded credentials clean code
⏺ Invoked skill speckit-constitution
⏺ Read ./.specify/memory/constitution.md (50 lines)
⏺ Read ./.specify/templates/plan-template.md (104 lines)
⏺ Read ./.specify/templates/spec-template.md (128 lines)
⏺ Read ./.specify/templates/tasks-template.md (251 lines)
[skill executes successfully and writes the constitution]
```

Devin recognized the skill, read project context from AGENTS.md, executed the workflow, and wrote back to .specify/memory/constitution.md. End-to-end working.

Checklist

• Tests added (tests/integrations/test_integration_devin.py)
• All tests pass locally (uv run pytest)
• Lint passes (uvx ruff check src/)
• Tested with actual Devin CLI

AI Disclosure

Used Claude (Cascade in Windsurf IDE) to scaffold the integration class, test file, and PR description. All code follows existing patterns from the Codex/Kimi skills integrations. Manual testing with the real Devin CLI was performed by the author.