meta: Use mise everywhere and clean up contributing docs

[Abhijeet Prasad (AbhiPrasad)]

This PR consolidates and cleans up repo guidance, standardizes the developer workflow around mise, simplifies local makefiles, and updates CI to use mise consistently.

AI Summary

What changed

Docs and agent guidance

• Moved repo instructions into a single root AGENTS.md
• Replaced root CLAUDE.md and py/CLAUDE.md with symlinks to AGENTS.md
• Rewrote AGENTS.md to be optimized for AI agents
• Rewrote CONTRIBUTING.md to be optimized for human contributors
• Updated integration docs to align with the new mise-first workflow

Developer workflow and makefiles

• Made mise the primary setup path across docs and root commands
• Removed the old hand-rolled root venv bootstrap from Makefile
• Turned the root Makefile into a thin wrapper around py/Makefile
• Fixed root delegated targets to run under mise exec so they work in fresh shells without mise activate
• Improved root make help output and clarified that test-wheel requires a built wheel
• Simplified integrations/langchain-py/Makefile to use uv sync / uv run
• Cleaned up integrations/adk-py/examples/Makefile without changing behavior
• Reduced source env.sh encouragement across docs

Tool version management

• Made .tool-versions the single source of truth for uv
• Removed the duplicated uv pin from mise.toml
• Updated py/Makefile to read the uv version from .tool-versions

CI

• Migrated workflows from actions/setup-python, astral-sh/setup-uv, and pre-commit/action to jdx/mise-action@v3
• Updated CI commands to run through mise exec
• Kept explicit per-job Python selection for matrix jobs
• Fixed reusable ADK and LangChain workflows to install uv explicitly under mise-action

Validation

Ran locally from the repo root:

• make help
• make install-dev
• make install-deps
• make develop
• make test-core
• make lint
• make pylint
• make fixup

Notable results:

• Root make test-core passed through the delegated mise path
• Root make lint and make pylint passed
• Root make test-wheel correctly failed without a built wheel, matching its documented prerequisite

Also validated:

• updated workflow YAML parses successfully
• integration workflow fix now provisions uv where required

Notes

• make test / make nox still run the full SDK nox matrix and were confirmed to enter the correct delegated path
• pre-existing pytest warnings around pytest.mark.forked remain unchanged