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/session.rs

@@ -14,6 +14,7 @@ const ROTATE_AFTER_BYTES: u64 = 256 * 1024;
 const MAX_ROTATED_FILES: usize = 3;
 static SESSION_ID_COUNTER: AtomicU64 = AtomicU64::new(0);
 
+/// Speaker role associated with a persisted conversation message.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum MessageRole {
     System,
@@ -22,6 +23,7 @@ pub enum MessageRole {
     Tool,
 }
 
+/// Structured message content stored inside a [`Session`].
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum ContentBlock {
     Text {
@@ -40,6 +42,7 @@ pub enum ContentBlock {
     },
 }
 
+/// One conversation message with optional token-usage metadata.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ConversationMessage {
     pub role: MessageRole,
@@ -47,6 +50,7 @@ pub struct ConversationMessage {
     pub usage: Option<TokenUsage>,
 }
 
+/// Metadata describing the latest compaction that summarized a session.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct SessionCompaction {
     pub count: u32,
@@ -54,6 +58,7 @@ pub struct SessionCompaction {
     pub summary: String,
 }
 
+/// Provenance recorded when a session is forked from another session.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct SessionFork {
     pub parent_session_id: String,
@@ -65,6 +70,7 @@ struct SessionPersistence {
     path: PathBuf,
 }
 
+/// Persisted conversational state for the runtime and CLI session manager.
 #[derive(Debug, Clone)]
 pub struct Session {
     pub version: u32,
@@ -91,6 +97,7 @@ impl PartialEq for Session {
 
 impl Eq for Session {}
 
+/// Errors raised while loading, parsing, or saving sessions.
 #[derive(Debug)]
 pub enum SessionError {
     Io(std::io::Error),