Add integration-aware command hints

[nike-17]

Description

Adds integration-aware user-facing command invocation rendering so generated Spec Kit hints use the active agent syntax. Codex now renders $speckit-... hints, standard slash-command integrations keep their existing behavior, and shared templates/scripts, extension hooks, presets, and init next steps all route through the integration-owned display invocation API.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

Commands run:

uv run --extra test pytest tests/integrations tests/test_agent_config_consistency.py tests/test_workflows.py -q
uv run --extra test pytest tests/integrations/test_integration_codex.py tests/integrations/test_integration_base_skills.py -q
uv run --extra test pytest tests -k "codex and command" -q

Also initialized temporary Codex and Claude projects and verified Codex generated $speckit-plan / $speckit-git-commit without slash-form leaks, while Claude retained /speckit-plan.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Implemented with Codex assistance in this repository workspace.