fix(agent-core): refresh system prompt on resume when roleAdditional changes

[TennyZhuang]

This patch fixes the persisted-session resume path so that a fresh roleAdditional is applied to the system prompt.

Changes:

• Store the current session's roleAdditional in agent metadata.
• In resumePersistedAgent, after restoring the profile handle, compare the persisted roleAdditional with the current session's. If they differ, call agent.refreshSystemPrompt() and persist the updated metadata.
• Add regression tests covering:
  1. Re-rendering the system prompt with the fresh roleAdditional after resume.
  2. Keeping the persisted prompt intact when roleAdditional is unchanged.
  3. Leaving an unresolved custom profile unchanged when roleAdditional changes.

Target: 0.28 candidate head e47c5495; replacement exact head f634fb92.