Add extensions option to createAgentSession SDK

- Accept ExtensionFactory[] for inline extensions (merged with discovery)
- Mark preloadedExtensions as @internal (CLI implementation detail)
- Update sdk.md with inline extension example
- Update CHANGELOG

packages/coding-agent/docs/session.md

@@ -16,14 +16,15 @@ Sessions have a version field in the header:
 
 - **Version 1**: Linear entry sequence (legacy, auto-migrated on load)
 - **Version 2**: Tree structure with `id`/`parentId` linking
+- **Version 3**: Renamed `hookMessage` role to `custom` (extensions unification)
 
-Existing v1 sessions are automatically migrated to v2 when loaded.
+Existing sessions are automatically migrated to the current version (v3) when loaded.
 
 ## Type Definitions
 
 - [`src/core/session-manager.ts`](../src/core/session-manager.ts) - Session entry types
-- [`packages/agent/src/types.ts`](../../agent/src/types.ts) - `AgentMessage`, `Attachment`, `ThinkingLevel`
-- [`packages/ai/src/types.ts`](../../ai/src/types.ts) - `UserMessage`, `AssistantMessage`, `ToolResultMessage`, `Usage`, `ToolCall`
+- [`packages/agent-core/src/types.ts`](../../agent-core/src/types.ts) - `AgentMessage`
+- [`packages/ai/src/types.ts`](../../ai/src/types.ts) - `UserMessage`, `AssistantMessage`, `ToolResultMessage`, `Usage`, `ToolCall`, `ImageContent`, `TextContent`
 
 ## Entry Base
 
@@ -45,13 +46,13 @@ interface SessionEntryBase {
 First line of the file. Metadata only, not part of the tree (no `id`/`parentId`).
 
 ```json
-{"type":"session","version":2,"id":"uuid","timestamp":"2024-12-03T14:00:00.000Z","cwd":"/path/to/project"}
+{"type":"session","version":3,"id":"uuid","timestamp":"2024-12-03T14:00:00.000Z","cwd":"/path/to/project"}
 ```
 
 For sessions with a parent (created via `/branch` or `newSession({ parentSession })`):
 
 ```json
-{"type":"session","version":2,"id":"uuid","timestamp":"2024-12-03T14:00:00.000Z","cwd":"/path/to/project","parentSession":"/path/to/original/session.jsonl"}
+{"type":"session","version":3,"id":"uuid","timestamp":"2024-12-03T14:00:00.000Z","cwd":"/path/to/project","parentSession":"/path/to/original/session.jsonl"}
 ```
 
 ### SessionMessageEntry
@@ -89,8 +90,8 @@ Created when context is compacted. Stores a summary of earlier messages.
 ```
 
 Optional fields:
-- `details`: Compaction-implementation specific data (e.g., file operations for default implementation, or custom data for custom hook implementations)
-- `fromHook`: `true` if generated by a hook, `false`/`undefined` if pi-generated
+- `details`: Compaction-implementation specific data (e.g., file operations for default implementation, or custom data for extension implementations)
+- `fromHook`: `true` if generated by an extension, `false`/`undefined` if pi-generated
 
 ### BranchSummaryEntry
 
@@ -102,30 +103,30 @@ Created when switching branches via `/tree` with an LLM generated summary of the
 
 Optional fields:
 - `details`: File tracking data (`{ readFiles: string[], modifiedFiles: string[] }`) for default implementation, arbitrary for custom implementation
-- `fromHook`: `true` if generated by a hook
+- `fromHook`: `true` if generated by an extension
 
 ### CustomEntry
 
-Hook state persistence. Does NOT participate in LLM context.
+Extension state persistence. Does NOT participate in LLM context.
 
 ```json
-{"type":"custom","id":"h8i9j0k1","parentId":"g7h8i9j0","timestamp":"2024-12-03T14:20:00.000Z","customType":"my-hook","data":{"count":42}}
+{"type":"custom","id":"h8i9j0k1","parentId":"g7h8i9j0","timestamp":"2024-12-03T14:20:00.000Z","customType":"my-extension","data":{"count":42}}
 ```
 
-Use `customType` to identify your hook's entries on reload.
+Use `customType` to identify your extension's entries on reload.
 
 ### CustomMessageEntry
 
-Hook-injected messages that DO participate in LLM context.
+Extension-injected messages that DO participate in LLM context.
 
 ```json
-{"type":"custom_message","id":"i9j0k1l2","parentId":"h8i9j0k1","timestamp":"2024-12-03T14:25:00.000Z","customType":"my-hook","content":"Injected context...","display":true}
+{"type":"custom_message","id":"i9j0k1l2","parentId":"h8i9j0k1","timestamp":"2024-12-03T14:25:00.000Z","customType":"my-extension","content":"Injected context...","display":true}
 ```
 
 Fields:
 - `content`: String or `(TextContent | ImageContent)[]` (same as UserMessage)
-- `display`: `true` = show in TUI with purple styling, `false` = hidden
-- `details`: Optional hook-specific metadata (not sent to LLM)
+- `display`: `true` = show in TUI with distinct styling, `false` = hidden
+- `details`: Optional extension-specific metadata (not sent to LLM)
 
 ### LabelEntry
 
@@ -190,7 +191,7 @@ for (const line of lines) {
       console.log(`[${entry.id}] Custom (${entry.customType}): ${JSON.stringify(entry.data)}`);
       break;
     case "custom_message":
-      console.log(`[${entry.id}] Hook message (${entry.customType}): ${entry.content}`);
+      console.log(`[${entry.id}] Extension message (${entry.customType}): ${entry.content}`);
       break;
     case "label":
       console.log(`[${entry.id}] Label "${entry.label}" on ${entry.targetId}`);
@@ -220,18 +221,20 @@ Key methods for working with sessions programmatically:
 - `appendThinkingLevelChange(level)` - Record thinking change
 - `appendModelChange(provider, modelId)` - Record model change
 - `appendCompaction(summary, firstKeptEntryId, tokensBefore, details?, fromHook?)` - Add compaction
-- `appendCustomEntry(customType, data?)` - Hook state (not in context)
-- `appendCustomMessageEntry(customType, content, display, details?)` - Hook message (in context)
+- `appendCustomEntry(customType, data?)` - Extension state (not in context)
+- `appendCustomMessageEntry(customType, content, display, details?)` - Extension message (in context)
 - `appendLabelChange(targetId, label)` - Set/clear label
 
 ### Tree Navigation
 - `getLeafId()` - Current position
+- `getLeafEntry()` - Get current leaf entry
 - `getEntry(id)` - Get entry by ID
-- `getPath(fromId?)` - Walk from entry to root
+- `getBranch(fromId?)` - Walk from entry to root
 - `getTree()` - Get full tree structure
 - `getChildren(parentId)` - Get direct children
 - `getLabel(id)` - Get label for entry
 - `branch(entryId)` - Move leaf to earlier entry
+- `resetLeaf()` - Reset leaf to null (before any entries)
 - `branchWithSummary(entryId, summary, details?, fromHook?)` - Branch with context summary
 
 ### Context