feat: support SPECKIT_INTEGRATION_<KEY>_EXECUTABLE env var (#2743)

* Initial plan

* feat: support SPECKIT_INTEGRATION_<KEY>_EXECUTABLE env var override

Adds `IntegrationBase._resolve_executable()` which reads
`SPECKIT_INTEGRATION_<KEY>_EXECUTABLE` (hyphens→underscores, uppercased)
and falls back to `self.key` when unset or whitespace-only.

All concrete `build_exec_args()` implementations now call
`self._resolve_executable()` instead of hard-coding `self.key` or
`"agy"` as the first argv token:
- MarkdownIntegration, TomlIntegration, SkillsIntegration (base classes)
- CodexIntegration, DevinIntegration, OpencodeIntegration, HermesIntegration, AgyIntegration
- CopilotIntegration (overrides `_resolve_executable()` to fall back to
  the platform-specific `_copilot_executable()` default; also updates
  `dispatch_command()` to use `_resolve_executable()`)

Tests added to tests/integrations/test_extra_args.py covering:
- default (unset) falls back to key
- env var replaces first argv token
- whitespace-only env var is a no-op
- key hyphen→underscore normalisation
- cross-integration scoping (wrong key ignored)
- all override integrations (Codex, Devin, Opencode, Copilot)
- Copilot dispatch_command path
- EXECUTABLE and EXTRA_ARGS can be set simultaneously

See issue #2596."

* fix: complete docstring sentence in _resolve_executable

* test: generalize extra-args test comments for override coverage

* Fix stale Codex executable comment

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/specify_cli/integrations/devin/__init__.py

@@ -48,7 +48,7 @@ class DevinIntegration(SkillsIntegration):
         stdout instead of structured JSON. ``requires_cli=True`` is
         kept on the integration for tool detection.
         """
-        args = [self.key, "-p", prompt]
+        args = [self._resolve_executable(), "-p", prompt]
         self._apply_extra_args_env_var(args)
         if model:
             args.extend(["--model", model])