refactor: restore git skill worked examples, type-tighten TableHelper.observe()

[braden-w]

Two independent housekeeping items that don't warrant separate PRs given their small scope.

Git skill restoration

The compression in commits 4ca8844–0dadacd removed worked diagram examples, the 9-section PR structure, and contextual guidance from the git skill. In practice, the verbose version produces measurably better PR descriptions because the worked examples use our actual vocabulary (YKeyValue, TableHelper, composition trees with O(n) annotations) to set a quality bar that generic principles can't match.

This restores the full version, then makes two targeted cuts that are genuine redundancies:

• Duplicate changelog section (lines 439–464 were an exact copy of rules already at 107–134)
• "When to Use Diagrams" summary list (each diagram subsection header already states when to use it)

Net result: 595 lines, down from the original 631 but with all worked examples intact.

Type-tighten TableHelper.observe()

observe() previously passed Set<string> for changed IDs, forcing consumers with branded ID types to cast at every usage:

// Before — identity casts everywhere
tabs.observe((changedIds) => {
  for (const id of changedIds) {
    const parsed = parseTabId(id as TabCompositeId);  // unnecessary
  }
});

// After — branded types flow through
tabs.observe((changedIds) => {
  for (const id of changedIds) {
    const parsed = parseTabId(id);  // id is already TabCompositeId
  }
});

Changed the callback signature to ReadonlySet<TRow['id']>. One cast in infrastructure (table-helper.ts) replaces 3+ casts at consumer sites. ReadonlySet is covariant (no .add()), so ReadonlySet<TabCompositeId> remains assignable to ReadonlySet<string> for any consumer that doesn't use branded types.