Fix `README`, `CONTRIBUTING`, and `AGENTS` drift

Summary

The top-level docs have drifted from the code in ways that actively mislead:

CONTRIBUTING.md

• Claims nix develop provides the Rust toolchain — it doesn't (#598); also omits that quality gates need a nightly rustfmt, that smoke tests need Go, and that cargo test --all-features requires a running Docker daemon (eth2api's integration feature uses testcontainers) — a contributor without Docker gets opaque failures.
• Doesn't mention that the pre-push hook runs deny + machete + fmt + clippy + the full test suite.
• The profiling section fences TOML as ```rust and suggests dhat = "latest", which isn't a valid version requirement.

README.md

• The "Version compatibility" section is copy-pasted from Charon ("two given versions of Charon are compatible if…") and never states Pluto's own promise or explains that the workspace version tracks the charon parity target.
• No build/install/quickstart at all — nothing about the Dockerfile, the published image, or test-infra/; the examples list covers 6 of 11 examples.

AGENTS.md

• The crate listing is 8 crates out of date: missing consensus, eth1wrap, featureset, frost, infosync, parsigex, priority, ssz; tree diagram omits third_party/ (load-bearing for charon interop in CI) and scripts/.

Cross-cutting: user-visible "Charon" branding in Pluto help text and crate docs — cli.rs long_about strings ("Verify that Charon can…", "Creates a local charon cluster configuration"), pluto-cluster/pluto-tracing module docs titled "# Charon …". Each needs a deliberate keep-or-rename decision (file/flag names like charon-enr-private-key are compatibility surface and should stay).

crates/cli/README.md's inverted parity table is already tracked by #584 — don't duplicate it here, but note cli.rs#L38 says "pluto-enr-private-key" while the actual default filename is charon-enr-private-key; the code comment is the wrong one.

Proposed change

One pass per file fixing the enumerated items; add a minimal Build/Run/Test quickstart to the README.

Start by reading README.md, CONTRIBUTING.md, and AGENTS.md alongside the workspace crate tree, Dockerfile, test-infra/, and the referenced cli.rs and module documentation. Check the listed toolchain, hooks, profiling, compatibility, quickstart, crate listing, and Charon-branding claims against the repository; avoid duplicating cli README parity work tracked by #584. Done means the enumerated documentation and comment drift is corrected and the keep-or-rename decisions are reflected consistently.