fix: resolve 3 bugs - subagent model override, empty plan completion, deep task refusal

- #2741: Pass inheritedModel as fallback in subagent-resolver when user hasn't
  configured an override, ensuring custom provider models take priority
- #2648: Fix getPlanProgress to treat plans with 0 checkboxes as incomplete
  instead of complete (total > 0 && completed === total)
- #2779: Relax Hephaestus single-task guard to accept multi-step sub-tasks
  from Atlas delegation, only rejecting genuinely independent tasks

Fixes #2741, fixes #2648, fixes #2779

src/tools/delegate-task/constants.ts

@@ -261,12 +261,16 @@ You are NOT an interactive assistant. You are an autonomous problem-solver.
 4. DO NOT ask clarifying questions - the goal is already defined
 
 **Autonomous executor mindset**:
-- You receive a GOAL, not step-by-step instructions
+- You receive a GOAL. When the goal includes numbered steps or phases, treat them as one atomic task broken into sub-steps - NOT as separate independent tasks.
 - Figure out HOW to achieve the goal yourself
 - Thorough research before any action
 - Fix hairy problems that require deep understanding
 - Work independently without frequent check-ins
 
+**Single vs. multi-step context**:
+- Sub-steps of ONE goal (e.g., "Step 1: analyze X, Step 2: implement Y, Step 3: test Z" for a single feature) = execute all steps, they are phases of one atomic task.
+- Genuinely independent tasks (e.g., "Task A: refactor module X" AND "Task B: fix unrelated bug Y") = flag and refuse, require separate delegations.
+
 **Approach**:
 - Explore extensively, understand deeply, then act decisively
 - Prefer comprehensive solutions over quick patches