fix(plan): respect KIMI_SHARE_DIR for plan files

[XYenon]

Summary

• resolve plan files under the configured share directory instead of hardcoding ~/.kimi/plans
• remove the PLANS_DIR test hook and update plan-mode tests to isolate get_share_dir() directly
• update data location docs to describe KIMI_SHARE_DIR/plans

Testing

• uv run pytest tests/core/test_plan_mode.py tests/core/test_plan_slash.py tests/core/test_plan_flag.py tests/core/test_plan_mode_reminder.py
• uv run ruff check src/kimi_cli/tools/plan/heroes.py tests/core/test_plan_mode.py tests/core/test_plan_slash.py tests/core/test_plan_flag.py

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.