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>

docs/archive/2025-10-milestones/STATUS.md

@@ -23,7 +23,7 @@
 ### ✅ 75 Tests (100% Pass)
 1. **Unit Tests** (35 tests)
    - **Rate Limiting** (15 tests): can_make_call(), increment_call_counter(), edge cases
-   - **Exit Detection** (20 tests): test saturation, done signals, completion indicators, @fix_plan.md validation, error handling
+   - **Exit Detection** (20 tests): test saturation, done signals, completion indicators, fix_plan.md validation, error handling
 
 2. **Integration Tests** (40 tests)
    - **Loop Execution** (20 tests): response analyzer detection, circuit breaker states, full loop integration, exit signal detection