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/copilot/__init__.py

@@ -134,6 +134,18 @@ class CopilotIntegration(IntegrationBase):
             ),
         ]
 
+    def _resolve_executable(self) -> str:
+        """Return the Copilot CLI executable, respecting the env-var override.
+
+        Checks ``SPECKIT_INTEGRATION_COPILOT_EXECUTABLE`` first.  Falls
+        back to the platform-specific default from ``_copilot_executable()``
+        (``copilot.cmd`` on Windows, ``copilot`` elsewhere) so that
+        existing behaviour is preserved when the env var is unset.
+        """
+        env_name = "SPECKIT_INTEGRATION_COPILOT_EXECUTABLE"
+        override = os.environ.get(env_name, "").strip()
+        return override if override else _copilot_executable()
+
     def build_exec_args(
         self,
         prompt: str,
@@ -148,7 +160,7 @@ class CopilotIntegration(IntegrationBase):
         # Controlled by SPECKIT_COPILOT_ALLOW_ALL_TOOLS env var
         # (default: enabled).  The deprecated SPECKIT_ALLOW_ALL_TOOLS
         # is also honoured as a fallback.
-        args = [_copilot_executable(), "-p", prompt]
+        args = [self._resolve_executable(), "-p", prompt]
         self._apply_extra_args_env_var(args)
         if _allow_all():
             args.append("--yolo")
@@ -217,7 +229,7 @@ class CopilotIntegration(IntegrationBase):
             agent_name = f"speckit.{stem}"
             prompt = args or ""
 
-        cli_args = [_copilot_executable(), "-p", prompt]
+        cli_args = [self._resolve_executable(), "-p", prompt]
         # Honour SPECKIT_INTEGRATION_COPILOT_EXTRA_ARGS for real workflow
         # runs.  `dispatch_command` builds cli_args inline rather than
         # going through `build_exec_args`, so the hook must be invoked