docs: Add context usage reporter hook example

Add Example 6 showing how to create a hook that reports context/token
usage after each Claude response:

- Python script reads transcript_path to access conversation history
- Estimates tokens using ~4 chars/token heuristic
- Outputs one-line report: "Context: ~45k/200k tokens (77% remaining)"
- Documents both Stop and UserPromptSubmit hook configurations
- Explains limitations (estimate vs exact /context command)

openspec/changes/add-context-usage-hook-example/proposal.md

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+# Change: Add Context Usage Reporting Hook Example
+
+## Why
+
+Users want to monitor their context window usage during Claude Code sessions. The hooks documentation currently lacks a practical example showing how to create a hook that reports context/token usage after each user request. This is valuable for understanding when context is getting full and when auto-compaction might occur.
+
+## What Changes
+
+- Add a detailed example hook that reports context usage after each user prompt
+- The hook will read the transcript file and estimate token usage
+- Include step-by-step explanation of how the hook works
+- Document the limitations (estimation vs exact token counts)
+
+## Impact
+
+- **Affected specs**: hooks-documentation (add new example)
+- **Affected code**: `06-hooks/README.md` (add new example section)
+- **User impact**: Users gain a practical example for monitoring context usage

openspec/changes/add-context-usage-hook-example/specs/hooks-documentation/spec.md

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+# Hooks Documentation Specification
+
+## ADDED Requirements
+
+### Requirement: Context Usage Reporting Hook Example
+The hooks lesson SHALL include a detailed example showing how to create a hook that reports context/token usage after each user request.
+
+#### Scenario: User learns to create context monitoring hook
+- **WHEN** a user reads the context usage reporter example
+- **THEN** they find a complete Python script that reads the transcript file
+- **AND** they understand how to estimate token usage from conversation history
+- **AND** they see the configuration for UserPromptSubmit or Stop hooks
+- **AND** they understand the limitations of token estimation
+
+#### Scenario: Hook output format is documented
+- **WHEN** a user implements the context usage hook
+- **THEN** they can generate a one-line report showing used tokens and remaining capacity
+- **AND** they understand the report is an estimate based on transcript analysis

openspec/changes/add-context-usage-hook-example/tasks.md

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+# Tasks: Add Context Usage Reporting Hook Example
+
+## 1. Documentation Update
+- [x] 1.1 Add new example section "Context Usage Reporter" to 06-hooks/README.md
+- [x] 1.2 Write Python hook script that reads transcript and estimates tokens
+- [x] 1.3 Add configuration example for UserPromptSubmit hook
+- [x] 1.4 Document how transcript_path provides access to conversation history
+- [x] 1.5 Explain token estimation approach and limitations
+- [x] 1.6 Show sample output format for the one-line report