docs: add timestamp correlation guide to AGENT_PLAYBOOK

Document how agents can correlate context entries (tasks, learnings,
decisions) to sessions using timestamps:

- Timestamp format: YYYY-MM-DD-HHMM (consistent across all entry types)
- Session time bounds: start_time and end_time fields
- Correlation method: Check if entry timestamp falls within session bounds
- Use cases: Tracing decisions, understanding context, debugging

Updated both the project's AGENT_PLAYBOOK.md and the template.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: josealekhine

.context/AGENT_PLAYBOOK.md

@@ -60,6 +60,47 @@ These are updated throughout the session.
 
 See `.claude/hooks/auto-save-session.sh` for the implementation.
 
+## Timestamp-Based Session Correlation
+
+Context entries (tasks, learnings, decisions) include timestamps that allow
+you to determine which session created them.
+
+### Timestamp Format
+
+All timestamps use `YYYY-MM-DD-HHMM` format:
+- **Tasks**: `- [ ] Do something #added:2026-01-23-1430`
+- **Learnings**: `- **[2026-01-23-1430]** Discovered that...`
+- **Decisions**: `## [2026-01-23-1430] Use PostgreSQL`
+- **Sessions**: `**start_time**: 2026-01-23-1400` / `**end_time**: 2026-01-23-1530`
+
+### Correlating Entries to Sessions
+
+To find which session added an entry:
+
+1. **Extract the entry's timestamp** (e.g., `2026-01-23-1430`)
+2. **List sessions** from that day: `ls .context/sessions/2026-01-23*`
+3. **Check session time bounds**:
+   ```bash
+   grep -l "start_time.*2026-01-23-14" .context/sessions/*.md
+   ```
+4. **Match**: Entry timestamp should fall between session's start_time and end_time
+
+### Example
+
+Given a learning with timestamp `2026-01-23-1430`:
+```
+Session A: start_time: 2026-01-23-1000, end_time: 2026-01-23-1200  # Too early
+Session B: start_time: 2026-01-23-1400, end_time: 2026-01-23-1530  # ✓ Contains 1430
+Session C: start_time: 2026-01-23-1600, end_time: 2026-01-23-1800  # Too late
+```
+→ The learning was added during Session B.
+
+### When Timestamps Help
+
+- **Tracing decisions**: "Why did we decide X?" → Find the session that added it
+- **Understanding context**: Read the full session for the discussion that led to an entry
+- **Debugging issues**: Correlate when a learning was discovered with what was happening
+
 ## Session File Structure (Suggested)
 
 Adapt this structure based on session type. 

.context/TASKS.md

@@ -136,7 +136,7 @@
 - [x] Increase timestamp precision in formatDecision() — change from YYYY-MM-DD to YYYY-MM-DD-HHMM
 - [x] Add start_time field to session summary files — record when session began
 - [-] Add last_update_time field to session summary files — skipped: end_time provides session bounds; tracking live updates requires state persistence
-- [ ] Document timestamp correlation approach in AGENT_PLAYBOOK.md — explain how to correlate entries to sessions by time overlap
+- [x] Document timestamp correlation approach in AGENT_PLAYBOOK.md — explain how to correlate entries to sessions by time overlap
 
 ### Phase 13: Rich Context Entries `#priority:medium` `#area:cli`
 - [ ] Add --file flag to ctx add — read entry content from a file instead of CLI arg

internal/templates/AGENT_PLAYBOOK.md

@@ -56,6 +56,33 @@ ls .context/sessions/
 
 See `.claude/hooks/auto-save-session.sh` for the implementation.
 
+## Timestamp-Based Session Correlation
+
+Context entries (tasks, learnings, decisions) include timestamps that allow
+you to determine which session created them.
+
+### Timestamp Format
+
+All timestamps use `YYYY-MM-DD-HHMM` format:
+- **Tasks**: `- [ ] Do something #added:2026-01-23-1430`
+- **Learnings**: `- **[2026-01-23-1430]** Discovered that...`
+- **Decisions**: `## [2026-01-23-1430] Use PostgreSQL`
+- **Sessions**: `**start_time**: 2026-01-23-1400` / `**end_time**: 2026-01-23-1530`
+
+### Correlating Entries to Sessions
+
+To find which session added an entry:
+
+1. **Extract the entry's timestamp** (e.g., `2026-01-23-1430`)
+2. **List sessions** from that day: `ls .context/sessions/2026-01-23*`
+3. **Check session time bounds**: Entry timestamp should fall between session's start_time and end_time
+
+### When Timestamps Help
+
+- **Tracing decisions**: "Why did we decide X?" → Find the session that added it
+- **Understanding context**: Read the full session for the discussion that led to an entry
+- **Debugging issues**: Correlate when a learning was discovered with what was happening
+
 ## Session File Structure (Suggested)
 
 Adapt this structure based on session type. Not all sections are needed for every session.