[docs] Onboarding documentation cleanup pass

[JonnyTran]

Problem

Multiple onboarding documents are inconsistent or stale, blocking new contributors:

1. Python version mismatch — AGENTS.md:12 claims "Python 3.10+ (server) / 3.9+ (SDK)" but extralit-server/pyproject.toml and extralit/pyproject.toml require >=3.11. SDK CLAUDE.md says 3.9+ as well.
2. Broken Markdown fence — README.md:50 uses ```base instead of ```bash.
3. TBD placeholders — extralit/README.md:88-92 has "TBD" for "Create an extraction schema" and "Add documents and start extraction" — the core SDK workflow is undocumented.
4. Missing per-component AGENTS.md — root AGENTS.md:5-8 references extralit-server/AGENTS.md, extralit-frontend/AGENTS.md, extralit/AGENTS.md. None exist. Either create them or remove the references.
5. No /examples README — examples directory has no top-level guidance on which examples are maintained, what they demonstrate, or which component they target.
6. No canonical dev environment doc — Tilt, docker-compose, and raw k8s YAMLs in examples/deployments/k8s/ all coexist without CONTRIBUTING.md indicating which to use for what.

Acceptance criteria

• All Python version claims agree with pyproject.toml.
• README fence renders correctly.
• SDK README has working end-to-end walkthrough (no TBD).
• Per-component AGENTS.md exist or the root reference is removed.
• examples/README.md lists each example with one-line purpose + which component it targets.
• CONTRIBUTING.md contains a "Choose a dev environment" section.