specs(collector,copilot): mark investigation complete and update Copilot collector array-value spec

- Change Phase 1 to "Investigation ✅ Complete" and record findings (reproduction, failing file, root cause)
- Update Phase 2 to "Ready to start" and expand success criteria and timeline
- Move and expand Investigation Findings section with format evolution details and impact
- Add concrete file/line references and test-file details for follow-up implementation/tests

Author: tikazyq

specs/20251102/002-copilot-collector-array-value-support/README.md

@@ -110,14 +110,15 @@ if err := json.Unmarshal(item.Value, &strValue); err == nil {
 
 ## Implementation Plan
 
-### Phase 1: Quick Fix 🚀 **Immediate** (2-4 hours)
+### Phase 1: Investigation ✅ **Complete**
 
-- [ ] Add error handling to skip unparseable items gracefully
-- [ ] Log warnings with file path and item kind for investigation
-- [ ] Test with failing file - verify no crashes
-- [ ] Release to unblock data collection
+- [x] Reproduced error with current struct definition
+- [x] Identified exact file and response item causing failure
+- [x] Analyzed pattern: Empty thinking steps use `[]` vs `""`
+- [x] Confirmed scope: Only 1 file affected (Oct 28, 2025 session)
+- [x] Root cause: Line 80 `Value string` + newer Claude Sonnet 4.5 format
 
-### Phase 2: Full Support 📋 **Follow-up** (1-2 days)
+### Phase 2: Full Support 📋 **Ready to start** (1-2 days)
 
 - [ ] Update `CopilotResponseItem` struct to use `json.RawMessage` for `Value`
 - [ ] Add `Content` field with nested value support
@@ -130,35 +131,70 @@ if err := json.Unmarshal(item.Value, &strValue); err == nil {
 
 ## Success Criteria
 
-- [x] All 63 chat files parse successfully (0 errors)
-- [x] Events extracted from previously failing file
-- [x] Backward compatible - existing 62 files still work
-- [ ] Tests cover string, array, and nested value formats
+**Phase 1 (Investigation) - ✅ Complete:**
+
+- [x] Confirmed root cause: `Value string` cannot handle array type
+- [x] Identified pattern: Empty thinking steps use `[]` instead of `""`
+- [x] Confirmed single file affected: Only latest Claude Sonnet 4.5 session
+- [x] Understood format evolution: New `id` field + array placeholder for empty content
+
+**Phase 2 (Implementation) - 🔨 Ready to start:**
+
+- [ ] All 63 chat files parse successfully (0 errors)
+- [ ] Events extracted from previously failing file
+- [ ] Backward compatible - existing 62 files still work
+- [ ] Tests cover string, array, and empty array formats
 - [ ] Zero parsing errors in production backfill
 
 ## Timeline
 
 **Estimated Effort**:
 
-- Phase 1: 2-4 hours
-- Phase 2: 1-2 days
+- Phase 1 (Investigation): ✅ Complete (~2 hours)
+- Phase 2 (Implementation): 1-2 days
 
-## References
+## Investigation Findings
+
+**Why this file is different from parseable ones:**
+
+Out of 63 Copilot chat session files, **only this one** has array-typed values:
 
-- [Related Spec](../path/to/spec)
-- [Documentation](../../../docs/something.md)
+| Aspect              | Most Files (62)           | Failing File (1)                          |
+| ------------------- | ------------------------- | ----------------------------------------- |
+| **Date**            | Oct 18-31, 2025           | Oct 28, 2025 (most recent)                |
+| **Model**           | Various Copilot models    | `copilot/claude-sonnet-4.5`               |
+| **Thinking format** | `value: "string content"` | `value: []` (empty array) + `id` field    |
+| **Pattern**         | String values only        | Mixed: strings + empty array              |
+| **New field**       | No `id` field             | 412-char encrypted `id` on thinking items |
+
+**Root cause of the difference:**
+
+1. **Format evolution**: Claude Sonnet 4.5 introduced extended thinking format
+2. **New fields**: Added encrypted `id` field (412 chars) to thinking items
+3. **Array placeholder**: Empty thinking steps use `value: []` instead of `value: ""`
+4. **Backward compatibility**: Most thinking items still use string format
+5. **Edge case**: Only affects empty thinking steps in newest sessions
+
+This represents a **breaking format change** that will become more common as users upgrade to Claude Sonnet 4.5.
+
+## References
 
 ### Files to Modify
 
-- `packages/collector/internal/adapters/copilot_adapter.go` - Struct definition and parsing logic
-- `packages/collector/internal/adapters/copilot_adapter_test.go` - Add test cases (to be created)
+- `packages/collector/internal/adapters/copilot_adapter.go` - Line 80: Change `Value string` to `json.RawMessage`
+- `packages/collector/internal/adapters/copilot_adapter.go` - Update parsing logic in `extractToolAndResponseEvents()`
+- `packages/collector/internal/adapters/copilot_adapter_test.go` - Add test cases for array values
 
 ### Test Files
 
-- Failed file: `/Users/marvzhang/Library/Application Support/Code - Insiders/User/workspaceStorage/5987bb38e8bfe2022dbffb3d3bdd5fd7/chatSessions/571316aa-c122-405c-aac7-b02ea42d15e0.json`
-- Working files: Any of the other 62 successfully parsed files
+- **Failed file**: `571316aa-c122-405c-aac7-b02ea42d15e0.json` (Oct 28, 2025, Claude Sonnet 4.5 session)
+  - Location: VS Code Insiders workspace storage
+  - Contains: 7 requests, 1 array value at response item #28
+  - Pattern: `{kind: "thinking", value: [], id: "...412 chars..."}`
+- **Working files**: Any of the other 62 successfully parsed files (all have string-only values)
 
 ### Related Issues
 
 - Backfill output showing: `Processed: 2960, Skipped: 0, Errors: 1`
 - Current stats: 2,930/2,960 events imported (99% success)
+- Format change: Claude Sonnet 4.5 extended thinking with encrypted `id` field