docs: describe the runtime public API surface

This adds crate-level and type-level Rustdoc to the runtime crate's core exported types so downstream crates and contributors can understand the session, prompt, permission, OAuth, usage, and tool I/O primitives without spelunking every implementation file.

Constraint: The docs pass needed to stay focused on public runtime types without changing behavior
Rejected: Add blanket docs to every public item in one sweep | larger churn than needed for a targeted docs pass
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: When exporting new runtime primitives from lib.rs, add a short Rustdoc summary in the defining module at the same time
Tested: cargo build --workspace; cargo test --workspace
Not-tested: rustdoc HTML rendering beyond  doc-test coverage

rust/crates/runtime/src/bash.rs

@@ -14,6 +14,7 @@ use crate::sandbox::{
 };
 use crate::ConfigLoader;
 
+/// Input schema for the built-in bash execution tool.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub struct BashCommandInput {
     pub command: String,
@@ -33,6 +34,7 @@ pub struct BashCommandInput {
     pub allowed_mounts: Option<Vec<String>>,
 }
 
+/// Output returned from a bash tool invocation.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct BashCommandOutput {
     pub stdout: String,
@@ -64,6 +66,7 @@ pub struct BashCommandOutput {
     pub sandbox_status: Option<SandboxStatus>,
 }
 
+/// Executes a shell command with the requested sandbox settings.
 pub fn execute_bash(input: BashCommandInput) -> io::Result<BashCommandOutput> {
     let cwd = env::current_dir()?;
     let sandbox_status = sandbox_status_for_input(&input, &cwd);