feat(prompt): add public system prompt template override

[ngoclam9415]

Summary

• expose STARAgent.override_system_prompt_template(template, persist=False) as the public full-template replacement API
• keep overrides ephemeral and agent-scoped by default; preserve codec rendering behavior and allow codec repositories to persist overrides explicitly
• provide the supported API needed for HFC to remove its private prompt-field workaround
• harden shared-runtime concurrency, per-agent prompt isolation, cache invalidation, and persisted-template consistency
• document replacement semantics, persistence limits, and roadmap/changelog status

Test evidence

• 1,440 passed, 195 skipped, 1 xfailed for tests excluding tests/cli
• changed-file py_compile passed
• changed-file Ruff passed