fix(core): render granular instruction updates

[kitlangton]

What

Instruction changes now append only the affected file deltas instead of repeating every loaded AGENTS.md file in full.

For each modified file, OpenCode renders both a three-context unified diff and the complete new instructions, then uses whichever final message is shorter. Added and removed files use explicit messages.

Before / After

Before: Editing one line in one instruction file appended a complete replacement containing every global and project instruction file. The session learned the correct new state, but paid the token cost of unrelated unchanged files.

After: Unchanged files are omitted. A localized edit usually appends a small diff, while a rewrite or tiny file appends the complete new instructions when that is shorter. Additions and removals remain explicit.

How

• packages/core/src/instruction-discovery.ts compares the previous and current file arrays by path.
• Modified files use the existing diff dependency to produce unified patches with three lines of context.
• The renderer compares the complete diff and replacement messages and emits the shorter one.
• Added files include their complete contents; removed files name the source that no longer applies.
• packages/core/test/instruction-discovery.test.ts covers diff selection, replacement selection, and the real watcher-driven update/delete flow.

Scope

This does not attempt to attribute file changes to the session that made them or suppress self-authored updates. Durable instruction state, chronological updates, and compaction behavior are unchanged.

Testing

• bun run test in packages/core: 1,709 passed, 16 skipped
• bun run test test/instruction-discovery.test.ts in packages/core: 10 passed
• bun typecheck in packages/core
• Repository pre-push typecheck: 34 tasks passed
• bunx prettier --check packages/core/src/instruction-discovery.ts packages/core/test/instruction-discovery.test.ts
• git diff --check

Flow

flowchart LR
  A[Read instruction files] --> B[Compare with admitted values]
  B --> C{Change type}
  C -->|Modified| D[Render diff and replacement]
  D --> E[Append shorter message]
  C -->|Added| F[Append new instructions]
  C -->|Removed| G[Append removal notice]
  E --> H[Durable chronological update]
  F --> H
  G --> H

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