feat(cc-013): add auto memory companion repo with three exercises

Companion repo for CC-013. Covers triggering memory, structuring
MEMORY.md with index and topic files, and proving cross-session
persistence shapes Claude behavior.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

exercises/03-cross-session.md

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+# Exercise 03: Cross-Session Persistence
+
+**Concept:** Auto Memory (CC-013)
+**Level:** Advanced
+**Time:** ~20 minutes
+**Builds on:** [Exercise 01](./01-trigger-memory.md) and [Exercise 02](./02-structure-memory.md)
+
+---
+
+## Objective
+
+Prove that memory persists across fully closed sessions and silently
+shapes Claude's behavior, without you repeating any preferences.
+
+This is the payoff exercise. You are not just creating memory files.
+You are watching them change how Claude writes code.
+
+---
+
+## The Scenario
+
+You have a coding preference. You tell Claude once. You close Claude
+Code entirely. You reopen it. You ask Claude to write code. Without
+mentioning the preference, you check whether Claude followed it.
+
+This is the pattern that makes auto memory genuinely useful: you
+teach Claude once, and it holds.
+
+---
+
+## Instructions
+
+**Step 1:** In a fresh Claude Code session, register a preference
+you can test in code. Paste this prompt:
+
+```
+Remember that I never use semicolons in TypeScript. No exceptions.
+Write this to the user preferences memory file.
+```
+
+Claude will save it. Confirm the save.
+
+**Step 2:** Close Claude Code completely.
+
+Not just `/clear`. Exit the process entirely:
+
+```
+/exit
+```
+
+Or close the terminal window and open a new one.
+
+**Step 3:** Reopen Claude Code in the same project directory.
+
+```bash
+cd claude-code-auto-memory
+claude
+```
+
+Do not mention the no-semicolons preference. Do not paste any context
+about your style. Just open it.
+
+**Step 4:** Ask Claude to write a TypeScript function. Paste this:
+
+```
+Write a TypeScript function called formatDate that takes a Date object
+and returns a string in YYYY-MM-DD format.
+```
+
+Read the output carefully. Count the semicolons.
+
+**Step 5:** Check whether memory was loaded.
+
+```
+What TypeScript preferences do you have on file for me?
+```
+
+Claude should list the no-semicolons rule and any preferences from
+earlier exercises, without you supplying them.
+
+**Step 6:** Add a project-level memory about this session.
+
+```
+Remember about this session: I completed all three auto memory exercises
+today. The project is a learning repo for CC-013. Next time, start by
+reading the exercise files to see what was covered.
+```
+
+This creates a lightweight session log. Open a new session and verify
+Claude greets you with context about what was done.
+
+---
+
+## What a Strong Solution Looks Like
+
+A strong result:
+
+- The `formatDate` function contains no semicolons, even though you
+  never mentioned the preference in this session
+- Claude lists your preferences accurately when asked in Step 5
+- The session note from Step 6 appears in memory and loads next session
+
+A result that misses the point:
+
+- Claude produces code with semicolons because the memory was not loaded
+- Claude says it has no preferences on file (likely a path or session issue)
+- The session note is never saved to a file
+
+---
+
+## Troubleshooting
+
+**Claude does not follow the preference:** Check that you are in exactly
+the same directory as Step 1. Run `pwd` and compare. The project hash
+is path-specific.
+
+**Memory shows nothing from earlier exercises:** If you used `/clear`
+instead of fully exiting and reopening, that may not have reset the
+session the same way. Try a full exit and reopen.
+
+**The function has semicolons despite the rule:** This can happen if
+the memory file was saved but Claude did not load it. Ask Claude:
+"Read the memory directory and tell me every file in it." If it is
+empty, the save in Step 1 did not complete.
+
+---
+
+## Challenge
+
+If you finish early or want to go further:
+
+- **Challenge A:** Register three conflicting preferences across two sessions
+  and see how Claude resolves them. Which one wins?
+- **Challenge B:** Delete one topic file manually from `~/.claude/projects/`
+  and verify Claude no longer applies that preference.
+- **Challenge C:** Set up memory in a different project on your machine and
+  confirm the two projects have separate, independent memory stores.
+
+---
+
+## Reflection
+
+After completing this exercise, consider:
+
+- What preferences or context would be most valuable to store in your
+  real projects?
+- What is the difference between memory in CLAUDE.md (checked into git,
+  shared with the team) and memory in the project memory store (local only)?
+- Where does auto memory break down, and when would you use CLAUDE.md instead?
+
+There is no right answer to these questions. The reflection is for you.
+
+---
+
+## What You Learned
+
+- **Memory accumulates progressively:** Each session leaves a trace.
+  Claude becomes more tuned to your context over time.
+- **Silent behavior shaping:** You do not need to repeat preferences.
+  They are loaded automatically and applied without prompting.
+- **Machine-local scope:** Everything in `~/.claude/projects/` stays
+  on your machine. Moving to a new machine means rebuilding memory.
+  This is a tradeoff worth knowing.
+
+---
+
+## What Is Next
+
+You have now worked through all three exercises for Auto Memory.
+
+Return to the article at https://fromaitochitta.com/cc-013 for
+the broader context, or explore related concepts in the
+Claude Code Memory and Instructions series.