Remove improve-skill as it's obsolete (#4)

- Remove reference to abandoned improve-skill (the skill no longer
exists on authors repo -
mitsuhiko/agent-stuff@3cc092d)
  - Add two practical tips to the "Agent self-improvement" phase:
- Use harness memory features (/memory in Claude Code, Memories in
Cursor) to persist lessons across sessions
    - Try post-mortem diffs to spot recurring antipatterns

Author: sebryu

src/content/docs/becoming-productive.md

@@ -27,7 +27,8 @@ This is not a strict scheme you must follow exactly. Phases can be optional or m
 5. **Human review** - ultimately, you (a human) are responsible for the code. Invest some time in reviewing it so that (a) you know what’s happening and (b) you won’t waste reviewers’ time.
 6. **Agent self-improvement** - talk to your agent: How can you both improve your workflow? What lessons can you learn from recent work? Perhaps some AGENTS.md rule or a new skill needs to be created?
    1. _If you use Claude Code, try the `/insights` command._
-   2. _Check out [https://skills.sh/mitsuhiko/agent-stuff/improve-skill](https://skills.sh/mitsuhiko/agent-stuff/improve-skill)_
+   2. _Use your harness's memory feature (e.g., `/memory` in Claude Code, Memories in Cursor) to persist lessons learned across sessions._
+   3. _Try post-mortem diffs: ask the agent to compare its first attempt vs the final version and explain what it got wrong. Great for spotting recurring antipatterns._
 
 ### Red/green TDD