feat: Add project_context_generator and project_context_updater agent skill

[aasthabharill]

b/521646880
b/521646367

• Used this skill to add project_context.md files for sourcedb-to-spanner, spanner-to-sourcedb and datastream-to-spanner
• Good at generating Overview, technical details and architecture diagram
• Needs some manual knowledge transfer and iterations in AI Agent Tips section

[gemini-code-assist]

Summary of Changes

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This pull request introduces a structured approach to project documentation by adding two new agent skills. These skills automate the generation and maintenance of a project-context.md file, which serves as a central source of truth for project architecture, tech stacks, and developer workflows. By providing a template and clear operational checklists, these changes aim to improve onboarding efficiency and ensure that documentation remains accurate as the codebase evolves.

Highlights

• New Agent Skills: Introduced two new agent skills: project_context_generator for creating initial project documentation and project_context_updater for maintaining it.
• Documentation Standardization: Added a standardized project-context.template.md to ensure consistency in project documentation across different repositories.
• Skill Index Update: Updated the global .agents/skills_index.md to include the newly added project context management skills.

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[gemini-code-assist]

Code Review

This pull request introduces two new agent skills: project-context-generator and project-context-updater, along with their respective test plans, templates, and registration in the skills index. The feedback highlights several improvement opportunities, including automatically detecting the git repository root, scoping git and PR searches to the specific project directory in monorepos, avoiding absolute paths for portability, resolving the chicken-and-egg issue when referencing the current PR, and correctly handling updates to existing PRs rather than blindly calling gh pr create.