feat: agent auto-coordination mode (claim + fallback)

[pszymkowiak]

Problem

Today agents need explicit prompts telling them to use grit claim, handle Blocked, and retry. This is fragile — the agent might forget or misuse the CLI.

Proposal

Add a high-level command that handles coordination automatically:

grit assign -a agent-1 -i "implement math functions" --file src/MathService.cs

This would:

1. List available (unlocked) symbols in the file
2. Pick one and claim it
3. Output the worktree path and symbol name
4. If all symbols in the file are locked, wait or return an error

The agent only needs one command instead of: symbols → status → claim → handle blocked → retry.

Use case

Simplify the agent prompt from 10 lines of grit instructions to:

Run: grit assign -a <your-id> -i "<task>" --file <file>
Edit the assigned function in the worktree path.
Run: grit done -a <your-id>