docs: add ROADMAP, VERSIONING, and dependency policy for SEP-1730 compliance

[felixweinberger]

Adds three policy files required for SEP-1730 SDK tiering:

Motivation and Context

SEP-1730 defines requirements for SDK tier classification. These policy files address the roadmap, versioning policy, and dependency update policy requirements.

How Has This Been Tested?

Documentation-only change, no code changes.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Part 1 of 2 in a stack. See also the stacked PR for feature documentation.

Files added:

• ROADMAP.md — tracks Tier 1 compliance work, spec tracking, and SDK improvements
• VERSIONING.md — documents semver scheme, breaking change policy, release branches
• .github/dependabot.yml — adds pip ecosystem alongside github-actions for dependency updates