fix: consolidate all config/data paths to ~/.codecora/cora-code/

[ajianaz]

What

Unify all scattered config/data paths into a single directory: ~/.codecora/cora-code/

Before (3+ locations):

~/.cora/                          ← config.yaml, auth.toml
~/.cache/cora/reviews/            ← review cache
~/.codecora/cora-code/            ← graph DB, update cache (already correct)

After (single location):

~/.codecora/cora-code/
├── config.yaml                   ← (was ~/.cora/config.yaml)
├── auth.toml                     ← (was ~/.cora/auth.toml)
├── cora.db                       ← (already here)
├── update-cache.json             ← (already here)
└── cache/
    └── reviews/                  ← (was ~/.cache/cora/reviews/)

Why

Issue #511: Data scattered across 3+ locations is confusing for users, complicates backups, and fragments the directory structure. The ~/.codecora/cora-code/ path already existed for graph DB + update cache — this PR makes it the single home for ALL cora-code data.

How

1. cora_dir() in loader.rs: now returns data_dir::cora_data_dir() instead of ~/.cora/
2. cache_dir() in cache.rs: now returns cora_data_dir().join("cache/reviews") instead of ~/.cache/cora/reviews/
3. Auto-migration: On first call to each function, checks for legacy paths and migrates:
   • config.yaml, auth.toml, config.toml → copied (not moved — user can safely delete old files)
   • Cache .json files → moved (rename, since cache is ephemeral)
   • Old .migrated marker also copied to prevent TOML→YAML re-migration
4. Idempotent: .migrated-to-codecora marker file prevents re-running
5. Safe: migration failures are logged but do not crash the app

Testing

• cargo build — clean
• cargo test — 22/22 pass (16 unit + 6 config)
• cargo clippy — 0 warnings
• cargo fmt --check — clean
• Pre-commit cora review — passed

Related Issues

Fixes #511

Checklist

• Code follows project style (cargo fmt)
• No clippy warnings
• All tests pass
• Self-reviewed
• Migration is backward compatible (old files preserved)
• Migration is idempotent (marker file)