[FEATURE]: Incrementally update AGENTS.md to improve input cache hit rate #29584

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Under the current implementation, when the content of AGENTS.md is modified during the lifecycle of a session, it will be reloaded into the system prompt segment at the very beginning of the context, causing all subsequent tokens to miss the cache.

Tested Environment: OpenCode 1.15.10 with DeepSeek V4 Pro on Linux

Reference implementation: Referring to Claude Code's behavior, the content of CLAUDE.md is loaded into the system prompt only when the session is created. If CLAUDE.md is later modified, Claude Code would insert a <system-reminder> to indicate that the content of CLAUDE.md has been changed, rather than directly modifying the system prompt.