fix: replace misleading context budget percentages with scope guidance

The planner prompt had vague context window budget percentages that
don't reflect how agents actually work. Replaced with concrete
scope guidance (keep stories to ~10 files) which aligns with the
existing scope budgets in config.json.

prompts/planner/plan.md

@@ -9,20 +9,8 @@ When breaking a feature into stories, think about:
 ### Independence
 Each story should be independently deployable. After completing story N, the codebase should be in a valid, working state — even if the feature isn't fully built yet.
 
-### Context Window Fit
-A story must fit in a single AI context window (~100K tokens). This means:
-- Reading relevant existing code
-- Understanding the task
-- Implementing the change
-- Writing tests
-- Running quality checks
-- Committing
-
-Budget roughly:
-- 30% of context for reading/understanding
-- 40% for implementation
-- 20% for testing and quality
-- 10% for bookkeeping (prd.json, progress.md)
+### Scope
+A story must be completable in a single iteration. Keep each story focused — a handful of files modified, not a sweeping change across the whole codebase. If a story requires reading and modifying more than ~10 files, it's too big — split it.
 
 ### Failure Isolation
 If a story fails (evaluator rejects it), the next iteration should be able to retry it cleanly. Stories with too many moving parts are hard to retry because partial state is messy.