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/base.py

@@ -146,6 +146,28 @@ class IntegrationBase(ABC):
         """
         return None
 
+    def _resolve_executable(self) -> str:
+        """Return the executable for this integration's CLI tool.
+
+        Checks ``SPECKIT_INTEGRATION_<KEY>_EXECUTABLE`` first, allowing
+        operators to override the binary path without modifying the
+        integration configuration — useful when the tool is installed in
+        a non-standard location or a specific version must be pinned.
+        Hyphens in the integration key are replaced with underscores and
+        the key is uppercased so that, for example, ``kiro-cli`` maps to
+        ``SPECKIT_INTEGRATION_KIRO_CLI_EXECUTABLE``.
+
+        Falls back to ``self.key`` when the env var is unset or
+        whitespace-only so existing behaviour is unchanged.
+
+        See issue #2596.
+        """
+        env_name = (
+            f"SPECKIT_INTEGRATION_{self.key.upper().replace('-', '_')}_EXECUTABLE"
+        )
+        override = os.environ.get(env_name, "").strip()
+        return override if override else self.key
+
     def _apply_extra_args_env_var(self, args: list[str]) -> None:
         """Append `SPECKIT_INTEGRATION_<KEY>_EXTRA_ARGS` env-var value to *args*.
 
@@ -895,7 +917,7 @@ class MarkdownIntegration(IntegrationBase):
     ) -> list[str] | None:
         if not self.config or not self.config.get("requires_cli"):
             return None
-        args = [self.key, "-p", prompt]
+        args = [self._resolve_executable(), "-p", prompt]
         self._apply_extra_args_env_var(args)
         if model:
             args.extend(["--model", model])
@@ -983,7 +1005,7 @@ class TomlIntegration(IntegrationBase):
     ) -> list[str] | None:
         if not self.config or not self.config.get("requires_cli"):
             return None
-        args = [self.key, "-p", prompt]
+        args = [self._resolve_executable(), "-p", prompt]
         self._apply_extra_args_env_var(args)
         if model:
             args.extend(["-m", model])
@@ -1402,7 +1424,7 @@ class SkillsIntegration(IntegrationBase):
     ) -> list[str] | None:
         if not self.config or not self.config.get("requires_cli"):
             return None
-        args = [self.key, "-p", prompt]
+        args = [self._resolve_executable(), "-p", prompt]
         self._apply_extra_args_env_var(args)
         if model:
             args.extend(["--model", model])