Silence render-disposed spam, unbreak bundled skills, drop board wake

Three independently ugly bugs that all fed the same log flood:

* Electron's `WebFrameMain.send` writes every "Render frame was disposed"
  to stderr via `console.error`. When the worker fans a second
  agent-event past a reloading renderer, we produced thousands of
  identical lines and stalled the main loop on stderr I/O. Added a
  `mainFrame.detached` guard so the send is short-circuited before
  Electron's own catch block fires, and installed a targeted filter in
  `suppress-warnings.ts` (now actually imported from the entry point)
  so the specific `console.error("Error sending from webFrameMain:"…)`
  noise never reaches the terminal. Any other payload still surfaces.

* `dispatchAgentMessage` accepted the human-reader sentinel as a wake
  target when an agent replied to `__board__` — `wakeAgentIfPossible`
  then called `db.getAgent("__board__")`, threw, and the dispatcher
  kept repeating on every send. Strip `HUMAN_SENDER_ID` at
  `resolveMessageWakeTargets`, and keep a defense-in-depth guard in the
  heartbeat so a stray caller (future automation fan-out) cannot
  reintroduce the crash.

* Pi's skills loader rejects any SKILL.md whose frontmatter `name`
  diverges from its parent directory. The four bundled `my-writing-style`
  copies declared `name: writing-style-skill`, so every install emitted
  a warning and dropped the skill. They also lived as orphan gitlinks
  (mode 160000 without .gitmodules), and the global `*.md` gitignore
  hid every other SKILL.md from version control. Fixed the frontmatter
  to match the dir, converted the dangling submodules to plain trees,
  and allowlisted `builtin-skills/**/*.md` so a fresh clone carries the
  catalogue instead of silently losing it.

builtin-skills/software-engineer/clean-code/references/err-avoid-null.md

@@ -0,0 +1,78 @@
+---
+title: Avoid Returning and Passing Null
+impact: HIGH
+impactDescription: eliminates NullPointerException risk
+tags: err, null, optional, null-object
+---
+
+## Avoid Returning and Passing Null
+
+Returning null creates work for callers and invites errors. One missing null check leads to NullPointerException. Throw exceptions, return empty collections, or use the Special Case / Null Object pattern.
+
+**Incorrect (returning null):**
+
+```java
+public List<Employee> getEmployees() {
+    if (thereAreNoEmployees) {
+        return null;  // Caller must check for null
+    }
+    return employees;
+}
+
+// Every caller must do this
+List<Employee> employees = getEmployees();
+if (employees != null) {  // Easy to forget
+    for (Employee e : employees) {
+        pay(e);
+    }
+}
+
+// Passing null
+public double calculatePay(Employee employee, Money bonus) {
+    // What if bonus is null?
+}
+calculatePay(employee, null);  // What does null mean?
+```
+
+**Correct (avoid null entirely):**
+
+```java
+// Return empty collection instead of null
+public List<Employee> getEmployees() {
+    if (thereAreNoEmployees) {
+        return Collections.emptyList();
+    }
+    return employees;
+}
+
+// Clean caller code
+for (Employee e : getEmployees()) {
+    pay(e);
+}
+
+// Use Optional for truly optional values
+public Optional<Employee> findById(Long id) {
+    return Optional.ofNullable(employeeMap.get(id));
+}
+
+// Use Special Case object
+public interface BillingPlan {
+    Money getRate();
+}
+
+public class NullBillingPlan implements BillingPlan {
+    public Money getRate() {
+        return Money.ZERO;
+    }
+}
+
+// Explicit parameters instead of null
+public double calculatePay(Employee employee, Money bonus) { /* ... */ }
+public double calculatePayWithoutBonus(Employee employee) { /* ... */ }
+```
+
+**When null/nullable is acceptable:**
+- Language-level nullable types with compiler enforcement (Kotlin `?`, TypeScript `strictNullChecks`, Rust `Option<T>`) make null safe because the compiler forces handling. The problem is untracked nullability, not the concept itself.
+- Interop with APIs or frameworks that require null (e.g., JDBC, some serialization libraries).
+
+Reference: [Clean Code, Chapter 7: Error Handling](https://www.oreilly.com/library/view/clean-code-a/9780136083238/)