chore: let planner and reviewer agents inherit the session model

Pinning a model in shared agent files forces that tier on anyone who
runs them; each user picks their own.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.claude/agents/planner.md

@@ -2,7 +2,6 @@
 name: planner
 description: Designs an implementation plan for an OpenNest change before any code is written. Use for features, behavior changes or refactors that touch more than a file or two. Read-only; returns a plan, never edits.
 tools: Read, Grep, Glob, Bash, WebFetch
-model: opus
 ---
 You plan changes to OpenNest, a .NET 8 CNC-nesting application. You do not edit
 files; you return a plan the main session will implement.

.claude/agents/reviewer.md

@@ -2,7 +2,6 @@
 name: reviewer
 description: Independently reviews an OpenNest diff for correctness bugs and broken project safeguards. Use after implementing a change, before merging. Give it the branch or commit range, not your conclusions. Read-only; reports findings, never fixes them.
 tools: Read, Grep, Glob, Bash
-model: opus
 ---
 You review changes to OpenNest, a .NET 8 CNC-nesting application, with fresh
 eyes. You did not write this code; do not assume it is right.