fix: add encoding="utf-8" to plan_session_storage.py open() calls

[realmaxc]

The plan_session_storage.py file has 6 open() calls (lines 87, 95, 105, 139, 147, 154) that read and write JSON session files without specifying an encoding. On systems where the default locale is not UTF-8, plan sessions containing non-ASCII text (goal descriptions, user notes, candidate plan titles) will silently corrupt data or raise UnicodeDecodeError.

This adds encoding='"utf-8"' to all 6 calls across both PlanSessionStorage (create, get, list_all) and CandidatePlanStorage (create, get, list_by_session).

Testing: Added tests/test_pr_04_plan_session_encoding.py with 3 tests:

• Session round-trip with Chinese user notes
• list_all with mixed ASCII/non-ASCII notes (English, French, Chinese)
• Candidate plan round-trip with Chinese title and abstract

All 3 tests pass in the sandbox container.