Add AppKit support and databricks-agent-skills integration

[jamesbroadhead]

Summary

• Prefer AppKit for new Databricks apps: databricks-apps-python/SKILL.md (formerly databricks-apps/) now leads with AppKit (TypeScript + React SDK) as the recommended approach for new apps, with Python frameworks (Dash, Streamlit, etc.) demoted to an explicit alternative. Frontmatter name and H1 title updated to match.
• Rename local skill directory: databricks-skills/databricks-apps → databricks-skills/databricks-apps-python to avoid collision with the incoming agent skill of the same name.
• Pull in databricks/databricks-agent-skills: Both install.sh and install.ps1 now fetch and install skills from databricks/databricks-agent-skills (databricks, databricks-apps, databricks-jobs → installed as databricks-jobs-bundles, databricks-lakebase, databricks-pipelines). Uses a single GitHub API tree call per install to fetch all files recursively, including nested references/ subdirectories.
• Source:install-name rename syntax: AGENT_SKILLS supports source:install-name entries (e.g. databricks-jobs:databricks-jobs-bundles) to decouple the upstream repo path from the local install directory name — no changes needed in the upstream repo.
• Zero skill name overlaps across bundled, APX, and agent skill sets.
• Fix pre-existing bug: databricks-app-python was listed in SKILLS/profiles but the directory was always databricks-apps — now consistent as databricks-apps-python.

Test plan

• bash install.sh --list-skills shows agent skills with install names (not source:install-name raw entries)
• bash install.sh --skills-profile app-developer installs databricks-apps-python, databricks-app-apx, and agent skills databricks, databricks-apps, databricks-lakebase, databricks-pipelines
• bash install.sh --skills databricks-jobs-bundles correctly fetches and installs from skills/databricks-jobs/ in the agent-skills repo into a directory named databricks-jobs-bundles
• bash install.sh --skills-profile all installs all 39 skills with no directory conflicts
• Same checks for install.ps1 on Windows

This pull request was AI-assisted by Isaac.