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/prompt.rs

@@ -5,6 +5,7 @@ use std::process::Command;
 
 use crate::config::{ConfigError, ConfigLoader, RuntimeConfig};
 
+/// Errors raised while assembling the final system prompt.
 #[derive(Debug)]
 pub enum PromptBuildError {
     Io(std::io::Error),
@@ -34,17 +35,21 @@ impl From<ConfigError> for PromptBuildError {
     }
 }
 
+/// Marker separating static prompt scaffolding from dynamic runtime context.
 pub const SYSTEM_PROMPT_DYNAMIC_BOUNDARY: &str = "__SYSTEM_PROMPT_DYNAMIC_BOUNDARY__";
+/// Human-readable default frontier model name embedded into generated prompts.
 pub const FRONTIER_MODEL_NAME: &str = "Claude Opus 4.6";
 const MAX_INSTRUCTION_FILE_CHARS: usize = 4_000;
 const MAX_TOTAL_INSTRUCTION_CHARS: usize = 12_000;
 
+/// Contents of an instruction file included in prompt construction.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ContextFile {
     pub path: PathBuf,
     pub content: String,
 }
 
+/// Project-local context injected into the rendered system prompt.
 #[derive(Debug, Clone, Default, PartialEq, Eq)]
 pub struct ProjectContext {
     pub cwd: PathBuf,
@@ -81,6 +86,7 @@ impl ProjectContext {
     }
 }
 
+/// Builder for the runtime system prompt and dynamic environment sections.
 #[derive(Debug, Clone, Default, PartialEq, Eq)]
 pub struct SystemPromptBuilder {
     output_style_name: Option<String>,
@@ -184,6 +190,7 @@ impl SystemPromptBuilder {
     }
 }
 
+/// Formats each item as an indented bullet for prompt sections.
 #[must_use]
 pub fn prepend_bullets(items: Vec<String>) -> Vec<String> {
     items.into_iter().map(|item| format!(" - {item}")).collect()
@@ -401,6 +408,7 @@ fn collapse_blank_lines(content: &str) -> String {
     result
 }
 
+/// Loads config and project context, then renders the system prompt text.
 pub fn load_system_prompt(
     cwd: impl Into<PathBuf>,
     current_date: impl Into<String>,