refactor(naming): remove @ prefix from on-disk filenames (#131)

* refactor(naming): remove @ prefix from on-disk filenames

BREAKING CHANGE: Renames @fix_plan.md → fix_plan.md and @AGENT.md → AGENT.md

This change improves POSIX compliance and compatibility with command-line tools.
The @ prefix was originally used to avoid naming conflicts, but with the .ralph/
folder structure introduced in v0.10.0, this convention is no longer necessary.

Changes:
- Update all scripts to use new naming (fix_plan.md, AGENT.md)
- Update migration script to handle both old and new naming conventions
- Update templates to use new naming
- Update all documentation references
- Update all 420 tests to expect new naming (TDD approach)

Migration:
- Existing projects with @-prefixed files will be automatically renamed
  when running ralph-migrate
- Projects already using the new naming will continue to work unchanged

* docs(claude): update file naming conventions section

* fix(migrate): prevent orphaned @-prefixed files during migration

When both root/@fix_plan.md and .ralph/@fix_plan.md exist, the root file
now takes priority and the .ralph/@fix_plan.md is removed (backup exists).
This prevents orphaned legacy files after migration.

---------

Co-authored-by: Test User <test@example.com>

setup.sh

@@ -40,8 +40,8 @@ mkdir -p .ralph/{specs/stdlib,examples,logs,docs/generated}
 
 # Copy templates to .ralph/
 cp "$TEMPLATES_DIR/PROMPT.md" .ralph/
-cp "$TEMPLATES_DIR/fix_plan.md" .ralph/@fix_plan.md
-cp "$TEMPLATES_DIR/AGENT.md" .ralph/@AGENT.md
+cp "$TEMPLATES_DIR/fix_plan.md" .ralph/fix_plan.md
+cp "$TEMPLATES_DIR/AGENT.md" .ralph/AGENT.md
 cp -r "$TEMPLATES_DIR/specs"/* .ralph/specs/ 2>/dev/null || true
 
 # Initialize git