fix(setup): allow static.rust-lang.org; correct the rust-toolchain paths in the doc

[potiuk]

Two related defects in how the framework's own sandbox config describes and permits the lychee rust hook. Both originate in #501, which rewrote the annotated block in docs/setup/secure-agent-setup.md but applied only the domain removals to .claude/settings.json and tools/sandbox-lint/expected.json.

1. static.rust-lang.org is required but was never allowed

The lychee hook is language: rust with additional_dependencies: ["cli:lychee"], so prek bootstraps its own toolchain on first run. It downloads rustup-init from static.rust-lang.org:

// crates/prek/src/languages/rust/rustup.rs:115
let url = format!("https://static.rust-lang.org/rustup/dist/{triple}/{filename}");

That host was not in allowedDomains, so a contributor with a cold prek cache cannot build the hook env in-sandbox. *.crates.io was allowed, which is why this looked fine — crate deps resolve, but the toolchain that consumes them can never be fetched.

2. The doc over-stated the framework's home-directory reach

The annotated block claimed the framework needs ~/.rustup/ and ~/.cargo/ in both allowRead and allowWrite. It does not. prek redirects everything into its own cache:

~/.cache/prek/tools/rustup/rustup       # prek's own rustup + toolchains
~/.cache/prek/cache/cargo/registry/     # CARGO_HOME
~/.cache/prek/hooks/rust-*/bin/lychee   # the built binary

All under the ~/.cache/ entries that were already present. The user's own ~/.rustup / ~/.cargo are never read or written — verified on a machine where both exist and neither contains a lychee binary.

Since those four lines were never in the real .claude/settings.json, the doc has been describing a config that does not exist, and claiming more home-directory access than the framework actually takes.

Also

The doc's allowedDomains list omitted api.bitbucket.org, which the real file has. Added so the block matches what it claims to reproduce.

Verification

• sandbox-lint: OK (.claude/settings.json matches expected.json)
• prek run --files <the three changed files>: all green
• pytest tools/sandbox-lint: 111 passed

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Generated-by: Claude Code (Opus 5)