fix(session): preserve session ID after resume

[pengpengyi92]

Summary

• Fixes /resume <session_id> restoring messages without updating the runtime's active session ID.
• Carries the restored ID explicitly through CommandResult, then updates both the runtime bundle and engine tool metadata before the next turn.
• Adds an end-to-end regression test proving that the next prompt is saved back to the restored session instead of a new startup session.

Validation

• pytest tests/test_ui/test_runtime_plan_mode.py tests/test_commands/test_registry.py -k "resume or plan_mode" -q
• ruff check --select F src/openharness/commands/registry.py src/openharness/ui/runtime.py tests/test_ui/test_runtime_plan_mode.py
• uv run ruff check src tests scripts (uv is not available in the local Windows test environment)
• uv run pytest -q (the full local run reached 1094 passed / 11 skipped; remaining failures require subprocess, TTY, symlink, or timezone capabilities unavailable in the Windows sandbox)
• cd frontend/terminal && npx tsc --noEmit (frontend not touched)

Notes

• Related issue: Fixes [Bug]: resume creates new session #345
• Follow-up work: None. This intentionally does not change session storage format or /resume selection behavior.