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.