writing-plans: task right-sizing, Global Constraints header, per-task Interfaces blocks

Claims are fidelity and variance, not dollars (full attribution in the
superpowers-evals experiment log, 2026-06-11 L1 entry):
- Global Constraints header: 0/5 -> 5/5 adoption in micro-tests, exact
  values verbatim; makes constraints mechanically propagatable to briefs
  and reviewers (a version-floor violation class shipped because they
  weren't). The one fix wave in the elicited full runs was a version-floor
  catch this header enabled.
- Per-task Interfaces blocks: 0 -> 100% of tasks, exact signatures,
  within-plan consistent; removes the controller's per-dispatch interface
  re-derivation.
- Task right-sizing: 9.4 -> 8.4 mean tasks at svelte scale (kills
  standalone Types/README micro-tasks); no effect at small scale.
- End-to-end (opus-written plan executed under SDD): guidance plan ran 1
  fix wave vs control's 2-4 (control plan shipped a real Sierpinski bug);
  execution cost equal within noise.

skills/writing-plans/SKILL.md

@@ -33,6 +33,15 @@ Before defining tasks, map out which files will be created or modified and what
 
 This structure informs the task decomposition. Each task should produce self-contained changes that make sense independently.
 
+## Task Right-Sizing
+
+A task is the smallest unit that carries its own test cycle and is worth a
+fresh reviewer's gate. When drawing task boundaries: fold setup,
+configuration, scaffolding, and documentation steps into the task whose
+deliverable needs them; split only where a reviewer could meaningfully
+reject one task while approving its neighbor. Each task ends with an
+independently testable deliverable.
+
 ## Bite-Sized Task Granularity
 
 **Each step is one action (2-5 minutes):**
@@ -57,6 +66,13 @@ This structure informs the task decomposition. Each task should produce self-con
 
 **Tech Stack:** [Key technologies/libraries]
 
+## Global Constraints
+
+[The spec's project-wide requirements — version floors, dependency limits,
+naming and copy rules, platform requirements — one line each, with exact
+values copied verbatim from the spec. Every task's requirements implicitly
+include this section.]
+
 ---
 ```
 
@@ -70,6 +86,12 @@ This structure informs the task decomposition. Each task should produce self-con
 - Modify: `exact/path/to/existing.py:123-145`
 - Test: `tests/exact/path/to/test.py`
 
+**Interfaces:**
+- Consumes: [what this task uses from earlier tasks — exact signatures]
+- Produces: [what later tasks rely on — exact function names, parameter
+  and return types. A task's implementer sees only their own task; this
+  block is how they learn the names and types neighboring tasks use.]
+
 - [ ] **Step 1: Write the failing test**
 
 ```python