docs(sdk): add runnable examples and tighten README quick start

[undivisible]

SDK ergonomics pass for the rx4 embed surface.

• README: Quick start now leads with library cargo add and the gated binary cargo install before the capability list; repaired the malformed Docs section.
• examples/: new minimal_agent, custom_tool, sessions, and provider_agent (requires providers) plus an equilibrium-style examples/README.md.
• AGENTS.md: fixed stale praefectus version (0.4.1 -> 0.8.0), corrected the tool loadout description, added zkr-memory/personality rows.

Validation run locally:

• cargo build --examples
• cargo build --example provider_agent --features providers
• cargo run --example minimal_agent / custom_tool / sessions
• cargo clippy --examples --features providers
• cargo fmt --check

---

Note

Low Risk
Documentation and example-only changes with no runtime or security behavior changes.

Overview
SDK ergonomics for library hosts: the README now leads with library cargo add, an updated embed snippet (Policy::workspace_write, pointer to provider_agent), runnable example commands, a gated-binary install section, a What it exposes capability list, and a cleaned-up docs index.

New examples/: four small programs (minimal_agent, custom_tool, sessions, provider_agent) plus examples/README.md with run instructions; Cargo.toml declares provider_agent with required-features = ["providers"].

AGENTS.md housekeeping: praefectus 0.4.1 → 0.8.0, expanded builtin tool loadout description, and feature-table rows for zkr-memory and personality.

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