Clean up docs, add session format documentation

packages/coding-agent/docs/session.md

@@ -0,0 +1,78 @@
+# Session File Format
+
+Sessions are stored as JSONL (JSON Lines) files. Each line is a JSON object with a `type` field.
+
+## File Location
+
+```
+~/.pi/agent/sessions/--<path>--/<timestamp>_<uuid>.jsonl
+```
+
+Where `<path>` is the working directory with `/` replaced by `-`.
+
+## Entry Types
+
+### SessionHeader
+
+First line of the file. Defines session metadata.
+
+```json
+{"type":"session","id":"uuid","timestamp":"2024-12-03T14:00:00.000Z","cwd":"/path/to/project","provider":"anthropic","modelId":"claude-sonnet-4-5","thinkingLevel":"off"}
+```
+
+### SessionMessageEntry
+
+A message in the conversation. The `message` field contains an `AppMessage` (see [RPC.md](./RPC.md#message-types)).
+
+```json
+{"type":"message","timestamp":"2024-12-03T14:00:01.000Z","message":{"role":"user","content":"Hello","timestamp":1733234567890}}
+{"type":"message","timestamp":"2024-12-03T14:00:02.000Z","message":{"role":"assistant","content":[{"type":"text","text":"Hi!"}],"api":"anthropic-messages","provider":"anthropic","model":"claude-sonnet-4-5","usage":{...},"stopReason":"stop","timestamp":1733234567891}}
+{"type":"message","timestamp":"2024-12-03T14:00:03.000Z","message":{"role":"toolResult","toolCallId":"call_123","toolName":"bash","content":[{"type":"text","text":"output"}],"isError":false,"timestamp":1733234567900}}
+```
+
+### ModelChangeEntry
+
+Emitted when the user switches models mid-session.
+
+```json
+{"type":"model_change","timestamp":"2024-12-03T14:05:00.000Z","provider":"openai","modelId":"gpt-4o"}
+```
+
+### ThinkingLevelChangeEntry
+
+Emitted when the user changes the thinking/reasoning level.
+
+```json
+{"type":"thinking_level_change","timestamp":"2024-12-03T14:06:00.000Z","thinkingLevel":"high"}
+```
+
+## Type Definitions
+
+See [`src/session-manager.ts`](../src/session-manager.ts) for entry types and [`packages/agent/src/types.ts`](../../agent/src/types.ts) for `AppMessage`.
+
+## Parsing Example
+
+```typescript
+import { readFileSync } from "fs";
+
+const lines = readFileSync("session.jsonl", "utf8").trim().split("\n");
+
+for (const line of lines) {
+  const entry = JSON.parse(line);
+  
+  switch (entry.type) {
+    case "session":
+      console.log(`Session: ${entry.id}, Model: ${entry.provider}/${entry.modelId}`);
+      break;
+    case "message":
+      console.log(`${entry.message.role}: ${JSON.stringify(entry.message.content)}`);
+      break;
+    case "model_change":
+      console.log(`Switched to: ${entry.provider}/${entry.modelId}`);
+      break;
+    case "thinking_level_change":
+      console.log(`Thinking: ${entry.thinkingLevel}`);
+      break;
+  }
+}
+```