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

Builds on #1715's reference discipline: the two structures are framed as
the narrow exceptions for spec content subagents must see (they get the
plan or one task of it, never the spec). Exception wording micro-validated
as PRI-2173 arm B: adopts 6/6 with zero restatement leak (lowest spec-copy
rate of all arms, plans -15% bytes vs dev control); Global Constraints
header elicited 0/5->5/5 with verbatim values, Interfaces 0->100% signature
availability (L1 micros). Value = lens determinism + mechanical extraction
into task briefs and reviewer constraints blocks, plus fix-wave reduction
(1 vs 2-4 in L1 full runs); the values themselves survive reference
discipline by riding code blocks.

skills/writing-plans/SKILL.md

@@ -13,6 +13,8 @@ Assume they are a skilled developer, but know almost nothing about our toolset o
 
 **Plans reference the spec; they never restate, paraphrase, or summarize it.** The spec owns the WHAT and WHY — requirements, acceptance criteria, design decisions; the plan owns the HOW — tasks, files, code, commands. Cite it by path in the header and by section where a task needs context. Reference discipline never means skipping the spec: if brainstorming produced one, it exists and the plan cites it. No Placeholders still requires repeating code and commands WITHIN the plan; copying FROM the spec is different: a step that needs a requirement's prose is under-specified — turn it into a concrete action. Snapshotting spec text into the plan hides drift, not prevents it. "Zero context" means each step is mechanically executable, not that the plan repeats the spec.
 
+**Two narrow exceptions to reference discipline** — subagents executing the plan see the plan (or a single task of it), never the spec, so two kinds of spec content travel in the plan itself: the `## Global Constraints` section (the spec's project-wide requirements, exact values copied verbatim) and each task's `**Interfaces:**` block (exact signatures). Copy those values exactly; everything else stays referenced, never restated.
+
 **Announce at start:** "I'm using the writing-plans skill to create the implementation plan."
 
 **Context:** If working in an isolated worktree, it should have been created via the `superpowers:using-git-worktrees` skill at execution time.
@@ -35,6 +37,15 @@ 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):**
@@ -61,6 +72,13 @@ 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.]
+
 ---
 ```
 
@@ -74,6 +92,12 @@ 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