feat: add native Cline integration

[pedropalb]

Description

This PR introduces a new integration for Cline (IDE-based agent).

• New Cline Integration: Added a dedicated integration for Cline in src/specify_cli/integrations/cline/. It uses the .clinerules/ directory and defaults to hyphenated command invocations (e.g., /speckit-plan) which Cline handles optimally.
• Unified Naming Convention: Refactored CommandRegistrar to respect a format_name callback in integration configurations. This unifies naming for hyphenated agents so that even extension commands (e.g., those from the git extension) use hyphenated filenames (e.g., speckit-git-feature.md) instead of dot-notation.
• Hook Rendering Update: Updated HookExecutor to correctly render hyphenated invocations for Cline projects.
• Documentation: Added Cline to the supported integrations documentation.
• Forge Compatibility: Updated Forge test expectations to reflect the new unified naming convention for extension commands.

Testing

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

AI Disclosure

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

Code generated with Gemini CLI with little manual tunings.