[FEATURE] Preserve file-change system-reminders across context summarization

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When working on a multi-file codebase over a long session, Claude Code automatically injects system-reminder diffs whenever a file is modified externally (e.g., by the user, an IDE formatter, or a linter). This works well within a single un-summarized context window.

However, when the conversation grows long enough to trigger context summarization, those file-change reminders are lost. After summarization, Claude has no awareness of external file modifications that were notified before the summary cutoff — even if those changes are directly relevant to ongoing work.

Proposed Solution

Before summarization occurs, preserve the most recent known state (or diff) of each file that has been notified via system-reminder during the session. After summarization, reapply those saved states to the new context — so Claude's awareness of external file modifications survives the summarization boundary.

This is analogous to how a text editor retains its file buffers across an undo-history truncation. The goal is not to replay every intermediate diff, but to carry forward the latest known state of each active file into the post-summarization context.

Alternative Solutions

My current workaround:
Since I don't know when a summarization will take place, immediately after any external change to source, I tell Claude to reread the changed source file or files. Also, I often just run my file backup - especially if there were many changes.

1) I assume this action is unnecessarily increasing my usage/cost.
2) I occasionally forget or miss a file, resulting in lost work/changes.
3) Increases the cognitive load of software development.

Priority

High - Significant impact on productivity

Feature Category

File operations

Use Case Example

Example scenario:

I modify a file externally (e.g., in my external product-specific IDE/editor/file-compare tool).
Claude receives the system-reminder diff and is aware of the change.
Conversation grows long; context is summarized.
Claude is now blind to that prior external change. Any subsequent edits may be based on a stale understanding of the file.

Additional Context

_No response_