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>

tests/helpers/fixtures.bash

@@ -123,7 +123,7 @@ You are Ralph, an autonomous AI development agent working on a Task Management A
 
 ## Current Objectives
 1. Study specs/* to learn about the project specifications
-2. Review @fix_plan.md for current priorities
+2. Review fix_plan.md for current priorities
 3. Implement the highest priority item using best practices
 4. Use parallel subagents for complex tasks (max 100 concurrent)
 5. Run tests after each implementation
@@ -134,7 +134,7 @@ You are Ralph, an autonomous AI development agent working on a Task Management A
 - Search the codebase before assuming something isn't implemented
 - Use subagents for expensive operations (file searching, analysis)
 - Write comprehensive tests with clear documentation
-- Update @fix_plan.md with your learnings
+- Update fix_plan.md with your learnings
 - Commit working changes with descriptive messages
 
 ## 🧪 Testing Guidelines (CRITICAL)
@@ -145,13 +145,13 @@ You are Ralph, an autonomous AI development agent working on a Task Management A
 - Focus on CORE functionality first, comprehensive testing later
 
 ## Current Task
-Follow @fix_plan.md and choose the most important item to implement next.
+Follow fix_plan.md and choose the most important item to implement next.
 EOF
 }
 
-# Sample @fix_plan.md
+# Sample fix_plan.md
 create_sample_fix_plan() {
-    local file=${1:-"@fix_plan.md"}
+    local file=${1:-"fix_plan.md"}
     local total=${2:-10}
     local completed=${3:-3}
 
@@ -203,9 +203,9 @@ EOF
 EOF
 }
 
-# Sample @AGENT.md
+# Sample AGENT.md
 create_sample_agent_md() {
-    local file=${1:-"@AGENT.md"}
+    local file=${1:-"AGENT.md"}
     cat > "$file" << 'EOF'
 # Agent Build Instructions
 
@@ -260,7 +260,7 @@ Created the following files:
 Running tests...
 ✓ All tests passed (5/5)
 
-Updating @fix_plan.md...
+Updating fix_plan.md...
 Completed: Set up basic project structure
 
 Ready for next task.
@@ -365,8 +365,8 @@ create_test_project() {
 
     # Create Ralph files in .ralph/ subdirectory
     create_sample_prompt ".ralph/PROMPT.md"
-    create_sample_fix_plan ".ralph/@fix_plan.md" 10 3
-    create_sample_agent_md ".ralph/@AGENT.md"
+    create_sample_fix_plan ".ralph/fix_plan.md" 10 3
+    create_sample_agent_md ".ralph/AGENT.md"
 
     # Create state files in .ralph/
     echo "0" > .ralph/.call_count