feat(sdd): follow a task's declared implementation tier at dispatch

The skeleton-first task template carries a Tier field the planner marks
deliberately. Model Selection now follows it instead of re-deciding at
dispatch, the implementer template's model slot reads it at fill time, and
an explicit mechanical ruling overrides the mid-tier contract floor —
the planner has already ruled the deliverable fully specified.

Task templates without a Tier field are unaffected: every route falls
through to the existing Model Selection guidance.

skills/subagent-driven-development/SKILL.md

@@ -195,6 +195,10 @@ Use the least powerful model that can handle each role to conserve cost and incr
 
 **Mechanical implementation tasks** (isolated functions, clear specs, 1-2 files): use a fast, cheap model. Most implementation tasks are mechanical when the plan is well-specified.
 
+When a task carries a **Tier:** field, follow it — the planner already
+ruled: mechanical → the cheapest available model; judgment → a standard
+model. Do not re-litigate the tier at dispatch.
+
 **Integration and judgment tasks** (multi-file coordination, pattern matching, debugging): use a standard model.
 
 **Architecture and design tasks**: use the most capable available model.
@@ -216,7 +220,10 @@ most expensive — which silently defeats this section.
 **Turn count beats token price.** Wall-clock and context cost scale with how
 many turns a subagent takes, and the cheapest models routinely take 2-3× the
 turns on multi-step work — costing more overall. Use a mid-tier model as the
-floor for reviewers and for implementers working from prose descriptions.
+floor for reviewers and for implementers working from task contracts or
+prose descriptions — unless the task's Tier line says mechanical: the
+planner has already ruled the deliverable fully specified, so treat a
+mechanical-tier contract like spelled-out content.
 When the task's plan text contains the complete code to write, the
 implementation is transcription plus testing: use the cheapest tier for
 that implementer. Single-file mechanical fixes also take the cheapest tier.

skills/subagent-driven-development/implementer-prompt.md

@@ -5,8 +5,11 @@ Use this template when dispatching an implementer subagent.
 ```
 Subagent (general-purpose):
   description: "Implement Task N: [task name]"
-  model: [MODEL — REQUIRED: choose per SKILL.md Model Selection; an omitted
-         model silently inherits the session's most expensive one]
+  model: [MODEL — REQUIRED: when the brief carries a Tier line, set from it:
+         mechanical → the cheapest model the subagent tool offers; judgment →
+         a standard mid-tier model. Otherwise choose per SKILL.md Model
+         Selection. An omitted model silently inherits the session's most
+         expensive one]
   prompt: |
     You are implementing Task N: [task name]