docs: document branch setup workflow

[lewtun]

Summary: Adds a Git workflow note requiring new branches and worktrees to start after pulling latest main. Tests: uv run ruff check ., uv run ruff format --check .

[github-actions]

Claude finished @lewtun's task in 37s —— View job

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PR Review: docs: document branch setup workflow

No blocking issues — LGTM

• Read the diff and surrounding AGENTS.md context
• Assess correctness, consistency, and style fit

This is a documentation-only change: it adds a ## Git Workflow section to AGENTS.md:23-25 advising that new branches/worktrees start from an up-to-date main. No code, routing, auth, or test surface is touched, so the review focus areas (correctness/security/LiteLLM routing/agent loop/test coverage) don't apply here.

The addition is well-placed (between Development Checks and GitHub CLI), matches the existing heading + bullet style, and the guidance is sound. The ## Hugging Face Space Deploys block at AGENTS.md:46 already opens with git pull --ff-only origin main, so the new note is consistent with the rest of the file.

P2 (informational, optional): the new bullet says to switch to main and pull the latest changes but doesn't specify the fast-forward-only form used elsewhere in this file (git pull --ff-only origin main, AGENTS.md:46). Aligning the wording would keep the guidance uniform — non-blocking.

Nothing to fix. Ship it.