Clarify that contributors create agentic plans in issues, not pull requests

[Copilot]

CONTRIBUTING.md implied contributors could create PRs directly. The repository only accepts contributions via issues containing detailed agentic plans that maintainers assign to Copilot Agent.

Changes

• Added explicit warnings at document opening, Quick Start, and PR Process sections stating traditional PRs are disabled
• Removed fork repository step - contributors don't need repo forks
• Replaced Step 3 from "Create a Pull Request with GitHub Copilot Agent" to "Copilot Agent Will Be Assigned"
• Added comprehensive example showing proper agentic plan structure with implementation details, file paths, validation rules, and test cases
• Clarified workflow: Issue creation → Maintainer review → Agent assignment → PR creation by agent
• Updated "Why" section from "Why No Local Development?" to "Why No Traditional Pull Requests?" with rationale

Example Agentic Plan Structure

## Add support for custom MCP server timeout configuration

### Implementation Plan
1. **Update Schema** (`pkg/parser/schemas/frontmatter.json`):
   - Add `timeout` field (integer, 5-300 seconds, default: 30)

2. **Update Validation** (`pkg/workflow/mcp_validation.go`):
   - Validate timeout range with error format: "[what's wrong]. [expected]. [example]"

3. **Add Tests** (`pkg/workflow/mcp_validation_test.go`):
   - Valid: 5, 30, 300
   - Invalid: 0, 4, 301, 1000

4. **Update Documentation**: Add timeout examples and behavior

The document now consistently communicates that contributors craft plans, maintainers assign to agent, and agent creates PRs.

Original prompt

Update contributions.md file to explain that pull requests are not enabled. Instead users should craft a full agentic plan that will be assigned to copilot.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

Pull request overview

This PR updates CONTRIBUTING.md to clarify that traditional pull requests are not enabled for this repository. Contributors must create detailed agentic plans in GitHub issues, which maintainers review and assign to GitHub Copilot Agent for implementation.

Changes:

• Added explicit warnings throughout the document stating traditional PRs are disabled
• Removed the "Fork the Repository" step and restructured the Quick Start guide
• Replaced "Create a Pull Request with GitHub Copilot Agent" with "Copilot Agent Will Be Assigned" to reflect the actual workflow
• Added a comprehensive example showing proper agentic plan structure with implementation details, file paths, validation rules, and test cases
• Updated the "Why No Local Development?" section to "Why No Traditional Pull Requests?" with expanded rationale

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