fix(copilot): read VS Code chatSessions so Copilot Chat users see cost (#555)

docs/providers/copilot.md

@@ -1,23 +1,25 @@
 # Copilot
 
-GitHub Copilot Chat (CLI and VS Code extension transcripts).
+GitHub Copilot Chat (CLI, VS Code core chat sessions, and VS Code extension transcripts).
 
 - **Source:** `src/providers/copilot.ts`
 - **Loading:** eager (`src/providers/index.ts:3`)
-- **Test:** `tests/providers/copilot.test.ts` (401 lines)
+- **Test:** `tests/providers/copilot.test.ts`
 
 ## Where it reads from
 
-Two JSONL locations plus an optional OpenTelemetry SQLite source (see below). All
-discovered sources are walked on every run; results merge and dedupe.
+Three JSONL locations plus an optional OpenTelemetry SQLite source (see below). OTel is
+preferred when present; chatSessions are only discovered when no OTel source is found.
+Other discovered sources are walked on every run; results merge and dedupe.
 
 1. **Legacy CLI sessions:** `~/.copilot/session-state/`
-2. **VS Code transcripts:** `~/Library/Application Support/Code/User/workspaceStorage/<hash>/GitHub.copilot-chat/transcripts/` and equivalents on Windows / Linux
-3. **OTel SQLite store:** VS Code Copilot Chat's `agent-traces.db` (see the OTel section). Preferred when present because it carries full input / output / cache token counts; the JSONL sources only record output tokens.
+2. **VS Code core chat sessions:** `~/Library/Application Support/Code/User/workspaceStorage/<hash>/chatSessions/*.jsonl` plus `~/Library/Application Support/Code/User/globalStorage/emptyWindowChatSessions/*.jsonl` and equivalents on Windows / Linux
+3. **VS Code transcripts:** `~/Library/Application Support/Code/User/workspaceStorage/<hash>/GitHub.copilot-chat/transcripts/` and equivalents on Windows / Linux
+4. **OTel SQLite store:** VS Code Copilot Chat's `agent-traces.db` (see the OTel section). Preferred when present because it carries full input / output / cache token counts; legacy JSONL sources only record output tokens.
 
 ## Storage format
 
-JSONL in the first two locations (schemas differ; the parser switches by detecting which schema the first event uses), and a SQLite DB for the OTel source.
+JSONL in the first three locations (schemas differ; the parser switches by source type / event shape), and a SQLite DB for the OTel source. VS Code core chat sessions use a delta journal: `kind:0` sets the root object, `kind:1` writes a value at path `k`, and `kind:2` appends items to an array path.
 
 ## OpenTelemetry (OTel) source
 
@@ -26,6 +28,11 @@ breakdowns (input, output, cache-read, cache-creation) from it, which the JSONL
 not record. Discovery is skipped with `CODEBURN_COPILOT_DISABLE_OTEL=1`, and the DB path
 can be overridden with `CODEBURN_COPILOT_OTEL_DB`.
 
+If OTel discovery finds at least one source, workspace `chatSessions/*.jsonl` and
+`emptyWindowChatSessions/*.jsonl` are skipped. Those journals can mirror the same Copilot
+turns under IDs that do not match OTel turn IDs, so CodeBurn prefers the richer OTel data
+instead of trying to dedupe across stores.
+
 - **Requires Node 22+.** The OTel source uses the built-in `node:sqlite` module (the same
   backend as Cursor / OpenCode). On Node 20, or if the DB is missing / locked / corrupt /
   wrong-schema, OTel is skipped and the JSONL/transcript sources are used as a fallback.
@@ -43,7 +50,9 @@ None for the JSONL sources. The OTel source uses a durable cache (see above).
 
 ## Deduplication
 
-Per `messageId` in both formats (`copilot.ts:118` for legacy, `copilot.ts:245` for transcripts).
+Legacy JSONL and transcript sessions dedupe per `messageId`. Core chat sessions dedupe per `copilot-chatsession:<sessionId>:<requestId>`, and are not discovered when an OTel source is present.
+
+If a workspace hash contains at least one `chatSessions/*.jsonl` file, the provider skips that hash's legacy `GitHub.copilot-chat/transcripts/` directory. The core chat session journal is the modern token-bearing source for the same conversations, so reading both would inflate call counts.
 
 ## Model inference