feat: support SPECKIT_INTEGRATION_<KEY>_EXECUTABLE env var

[Copilot]

Operators had no way to override the CLI binary path for an integration without modifying integration configuration, making it difficult to pin a specific version or use a non-standard install location.

Changes

IntegrationBase._resolve_executable() (new method)

• Reads SPECKIT_INTEGRATION_<KEY>_EXECUTABLE; key is uppercased with hyphens→underscores (e.g. kiro-cli → SPECKIT_INTEGRATION_KIRO_CLI_EXECUTABLE)
• Whitespace-only/unset → falls back to self.key (no behaviour change for existing users)

All build_exec_args() implementations updated to call self._resolve_executable() as the first argv token:

• Base classes: MarkdownIntegration, TomlIntegration, SkillsIntegration
• Override integrations: CodexIntegration, DevinIntegration, OpencodeIntegration, HermesIntegration, AgyIntegration

CopilotIntegration overrides _resolve_executable() to check the env var first, then fall back to the platform-specific default (copilot.cmd on Windows, copilot elsewhere). dispatch_command() is updated alongside build_exec_args().

Example

# Use a specific claude binary without changing any config
SPECKIT_INTEGRATION_CLAUDE_EXECUTABLE=/opt/claude-beta/bin/claude speckit run ...

# Works for hyphenated keys too
SPECKIT_INTEGRATION_KIRO_CLI_EXECUTABLE=/usr/local/bin/kiro-cli speckit run ...

SPECKIT_INTEGRATION_<KEY>_EXECUTABLE and SPECKIT_INTEGRATION_<KEY>_EXTRA_ARGS are independent and can be used simultaneously.

[Copilot]

Pull request overview

Adds an operator-facing escape hatch to override which CLI binary an integration uses (without changing integration config), by introducing a shared IntegrationBase._resolve_executable() that reads SPECKIT_INTEGRATION_<KEY>_EXECUTABLE and wiring it into integration argv construction (with a Copilot-specific override to preserve copilot.cmd behavior on Windows).

Changes:

• Introduce IntegrationBase._resolve_executable() to resolve the executable from SPECKIT_INTEGRATION_<KEY>_EXECUTABLE (with hyphen→underscore normalization) and fall back to self.key for backward compatibility.
• Update base and overridden build_exec_args() implementations to use _resolve_executable() as argv[0]; update CopilotIntegration.dispatch_command() similarly, with Copilot overriding _resolve_executable() to preserve platform defaults.
• Extend the existing env-var test suite to cover the new executable override behavior alongside extra-args behavior.

Show a summary per file

File	Description
tests/integrations/test_extra_args.py	Expands tests to cover *_EXECUTABLE override and updates env cleanup/docstrings.
src/specify_cli/integrations/base.py	Adds _resolve_executable() and wires it into shared base build_exec_args() implementations.
src/specify_cli/integrations/codex/init.py	Uses _resolve_executable() for argv[0] in Codex override.
src/specify_cli/integrations/devin/init.py	Uses _resolve_executable() for argv[0] in Devin override.
src/specify_cli/integrations/opencode/init.py	Uses _resolve_executable() for argv[0] in Opencode override.
src/specify_cli/integrations/hermes/init.py	Uses _resolve_executable() for argv[0] in Hermes override.
src/specify_cli/integrations/copilot/init.py	Adds Copilot-specific _resolve_executable() and updates build/dispatch argv[0] usage.
src/specify_cli/integrations/agy/init.py	Uses _resolve_executable() for argv[0] in Agy override.

Copilot's findings

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• Files reviewed: 8/8 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 0 new