feat(integrations): add Devin for Terminal skills-based integration (#2364)

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

- Register DevinIntegration as a SkillsIntegration with .devin/skills/ layout
- Add catalog entry, docs row, and supported-agents listing
- Display /speckit-<command> hyphen syntax in init "Next Steps" panel
  (matches Claude/Cursor/Copilot skills mode, since Devin invokes skills
  by directory name)

Closes #2346

* fix(devin): implement -p non-interactive dispatch; clarify skills comment

Addresses Copilot review on PR #2364:

- Override build_exec_args() in DevinIntegration to emit
  'devin -p <prompt> [--model X]' for non-interactive text dispatch
  (verified Devin CLI supports -p / --print). Returns None when
  output_json=True since Devin has no structured-output flag, so
  CommandStep workflows that require JSON cleanly raise
  NotImplementedError instead of crashing on an unknown CLI flag.
  requires_cli=True is retained for tool detection.

- Extend the skills-integrations enumeration comment in
  specify_cli/__init__.py to include copilot and devin so the
  comment matches the code below it.

* fix(devin): always return exec args; document plain-text stdout

Addresses third Copilot review comment on PR #2364.

Returning None from build_exec_args() when output_json=True
incorrectly used the codebase's IDE-only sentinel: workflow
CommandStep checks 'impl.build_exec_args("test") is None' to
detect non-dispatchable integrations (test_workflows.py exercises
this with WindsurfIntegration). The previous implementation made
Devin appear non-dispatchable to all command steps even though it
runs fine via 'devin -p'.

Always return the args list. When output_json is requested, Devin
is still dispatched and returns plain-text stdout instead of
structured JSON; the docstring documents this explicitly.

* docs(devin): include claude in skills-integrations enumeration comment

Addresses Copilot review on PR #2364: the comment listing skills
integrations omitted Claude, which is also a SkillsIntegration
subclass. Updated to keep the comment accurate for future readers.

* test(devin): add build_exec_args regression tests; bump catalog updated_at

Addresses Copilot review on PR #2364, per @mnriem's request to
'address the Copilot feedback, especially the testing ask':

- tests/integrations/test_integration_devin.py: add TestDevinBuildExecArgs
  with three regression assertions:
    * build_exec_args returns args (not the None IDE-only sentinel)
    * --output-format is never emitted, regardless of output_json
    * --model flag is passed through correctly
- integrations/catalog.json: bump top-level updated_at to reflect the
  Devin entry addition so downstream catalog consumers can detect the
  change reliably.

.github/ISSUE_TEMPLATE/agent_request.yml

@@ -8,7 +8,7 @@ body:
       value: |
         Thanks for requesting a new agent! Before submitting, please check if the agent is already supported.
         
-        **Currently supported agents**: Claude Code, Gemini CLI, GitHub Copilot, Cursor, Qwen Code, opencode, Codex CLI, Windsurf, Kilo Code, Auggie CLI, Roo Code, CodeBuddy, Qoder CLI, Kiro CLI, Amp, SHAI, Tabnine CLI, Antigravity, IBM Bob, Mistral Vibe, Kimi Code, Trae, Pi Coding Agent, iFlow CLI
+        **Currently supported agents**: Claude Code, Gemini CLI, GitHub Copilot, Cursor, Qwen Code, opencode, Codex CLI, Windsurf, Kilo Code, Auggie CLI, Roo Code, CodeBuddy, Qoder CLI, Kiro CLI, Amp, SHAI, Tabnine CLI, Antigravity, IBM Bob, Mistral Vibe, Kimi Code, Trae, Pi Coding Agent, iFlow CLI, Devin for Terminal
 
   - type: input
     id: agent-name