docs(prompt): enhance apply_diff tool instructions to improve Gemini model success rate (#619)

* docs(prompt): enhance apply_diff tool instructions for Gemini model success

* docs: add PR description for issue 611

* docs(plan): align PR description plan with actual changes by removing AGENTS.md reference

* docs(prompt): soften :start_line: to strongly recommended and untrack gitignored plan

Addresses PR #619 review feedback from edelauna:
- :start_line: is optional in the multi-search-replace parser, so describe it
  as strongly recommended rather than required (truthful while preserving
  compliance for smaller models).
- Remove plans/improve-apply-diff-prompt.pr.md from tracking; plans/ is
  gitignored.
- Align changeset summary with the corrected wording.

Author: awschmeder

.changeset/improve-apply-diff-prompt.md

@@ -0,0 +1,5 @@
+---
+"zoo-code": patch
+---
+
+Enhance the `apply_diff` tool description and parameter instructions to recommend `:start_line:` with exact syntax and emphasize copy-paste exact matching requirements, improving success rates for Gemini Flash and other smaller/faster models.

src/core/prompts/tools/native-tools/apply_diff.ts

@@ -2,14 +2,19 @@ import type OpenAI from "openai"
 
 const APPLY_DIFF_DESCRIPTION = `Apply precise, targeted modifications to an existing file using one or more search/replace blocks. This tool is for surgical edits only; the 'SEARCH' block must exactly match the existing content, including whitespace and indentation. To make multiple targeted changes, provide multiple SEARCH/REPLACE blocks in the 'diff' parameter. Use the 'read_file' tool first if you are not confident in the exact content to search for.`
 
-const DIFF_PARAMETER_DESCRIPTION = `A string containing one or more search/replace blocks defining the changes. The ':start_line:' is required and indicates the starting line number of the original content. You must not add a start line for the replacement content. Each block must follow this format:
+const DIFF_PARAMETER_DESCRIPTION = `A string containing one or more search/replace blocks defining the changes. The ':start_line:' is strongly recommended and indicates the starting line number of the original content. You must not add a start line for the replacement content. Each block must follow this format:
 <<<<<<< SEARCH
 :start_line:[line_number]
 -------
 [exact content to find]
 =======
 [new content to replace with]
->>>>>>> REPLACE`
+>>>>>>> REPLACE
+
+CRITICAL:
+- The ':start_line:[line_number]' header is strongly recommended for accurate matching. When provided, it must follow the exact syntax ':start_line:[integer]' (for example: ':start_line:220'). Do not write headers with shorthand forms like ':220' or variations like ':start_line=220'.
+- Copy the exact lines from the source file for a 100% string match including all whitespace, indentation, and newlines.
+- Ensure the separator '-------' is on its own line immediately following ':start_line:[line_number]' with a newline.`
 
 export const apply_diff = {
 	type: "function",