Canonicalize journal path to ./journal/ from project root
Replace the 3-tier JOURNAL_PATH configuration system (env var → .env →
platformdirs default) with a single canonical path: <project_root>/journal/.
Internal override _SOLSTONE_JOURNAL_OVERRIDE preserved for tests.
- Rewrite get_journal() and get_journal_info() in think/utils.py
- Remove platformdirs import and dependency
- Update all 80+ test files to use _SOLSTONE_JOURNAL_OVERRIDE
- Update all documentation to reference journal/ instead of $JOURNAL_PATH
- Update Makefile, .env.example, cortex subprocess env propagation
- Simplify sol config env output (just prints the path)
- Remove setup_cli() "JOURNAL_PATH not set" warning (no longer applicable)
Implements: cpo/specs/in-flight/journal-path-canonicalization.md