feat: collaborative workspace presence module with mock backend

[laurentketterle-hub]

Description

Lightweight collaborative workspace presence for multi-user IDE sessions.

Closes #30

What's included

• src/lappa/presence.py: Full presence service with UserPresence, WorkspaceState, and PresenceService
• tests/test_presence.py: 12 test cases covering all operations
• CI workflow (if missing)

Features

Feature	Description
Workspace management	Create, list, delete workspaces
User join/leave	Track users entering and leaving workspaces
Cursor tracking	Real-time cursor position per user (line, col, file)
Activity detection	30-second heartbeat timeout for active/inactive
User colors	Deterministic color assignment per user ID
Mock backend	In-memory store, no external dependencies

Design

• Mock backend acceptable per issue requirements
• Dataclass-based model for clean serialization
• UUID-based workspace IDs
• Thread-safe operations on in-memory state

Claim

I claim this bounty (#30).

[laurentketterle-hub]

/attempt 30

[laurentketterle-hub]

Friendly ping 👋 — this PR has been open for 4 days. Is there anything needed for review? Thanks!

[laurentketterle-hub]

👋 Friendly nudge — this PR has been open for a while. Anything blocking merge, or happy to adjust if review feedback is needed? Thanks!

[laurentketterle-hub]

/claim