docs(roadmap): define hosted distribution milestones

[fishmingyu]

Summary

Defines the hosted product stage around one reusable, commit-addressed repository-context artifact. The roadmap keeps the zero-credential route model-free, reflects GitHub Models' retirement, and separates an optional future managed embedding service from the open-source release contract.

Tracks #415.

Changes

• Preserve the completed local Developer Preview as the product baseline.
• Define H1-H6 for static export, local/BYO inference routes, Actions/Pages, artifact-backed MCP, quality-cost accounting, and release acceptance.
• Link each implementation issue and state the serverless Wiki security and capability boundary.
• Replace the retired GitHub Models dependency with local embeddings, explicit BYO endpoints, and a deterministic no-model route.
• Record a post-release managed semantic plane as optional infrastructure rather than a hidden OSS prerequisite.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring
• Performance improvement
• Tests

Testing

• Tests pass locally
• Added new tests for the changes

Validated with:

• pre-commit run --files docs/product_roadmap.md
• git diff --check
• mkdocs build --strict --site-dir /tmp/codenib-hosted-roadmap-site

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published

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