feat: add Claude Agent SDK implementation

[mwbrooks]

Summary

• Add a third parallel Casey implementation at claude-agent-sdk/ using the Claude Agent SDK
• Uses AsyncApp from Bolt for Python with fully async handlers
• Tools registered via @tool decorator and create_sdk_mcp_server() from the SDK
• Conversation context managed server-side via session IDs (no local message history storage)
• Updates root README, .claude/CLAUDE.md, CI matrix (ruff.yml), and dependabot.yml

Reviewers

# Checkout this branch
$ gh pr checkout 6

# Install the dependencies
$ cd claude-agent-sdk/
$ python3 -m venv .venv
$ source .venv/bin/activate
$ pip install -r requirements.txt

# Set ANTHROPIC_API_KEY
$ cp .env.sample .env
$ vim .env

# Run the app
$ slack run

# Open Slack
# -> Open "Casey - Claude Agent SDK (local)"
# -> Send a message in the DM
# -> Confirm it responds

Extra testing (for the bold an ambitious):

• App Home loads with category buttons
• Clicking a category opens the issue submission modal
• Submitting the modal starts a DM thread with Casey's response
• @casey mention in a channel produces a threaded response
• Follow-up messages in the same thread maintain conversation context
• Feedback buttons (thumbs up/down) work on responses

[zimeg]

@mwbrooks The frameworks expand! Thanks so much for bringing something honest to this collection of samples 🧪 ✨

[zimeg]

🧮 quibble: For @claude - Can you add alphabetical orderings to CLAUDE.md please?

[mwbrooks]

@zimeg good catch! Commit 51998f1 updates the ordering :)

[mwbrooks]

Thanks for the review @zimeg! I've alphabetized all the things and I think we're good bring this into main 🚀