fix(core): restore patch tool parity

[rekram1-node]

Summary

• restore V1 patch behavior while keeping the V2 patch tool name and vocabulary
• match parser acceptance, add overwrite, move, EOF-anchor, BOM, and whitespace behavior
• accept an initial update chunk without @@ and normalize CRLF patch delimiters without removing intentional carriage returns
• convert derivation exceptions into ordinary actionable tool failures
• compute each structured file diff once and derive both counts and formatted output from it
• update Codex and GPT system prompts to reference patch instead of apply_patch

Testing

• bun typecheck from packages/core
• bun test test/patch.test.ts test/tool-patch.test.ts from packages/core (26 passed)
• bun test test/patch.test.ts test/tool-patch.test.ts test/plugin/system-prompt.test.ts from packages/core before the syntax follow-up (30 passed)

Remaining Codex parity

• reject malformed headers, invalid hunk lines, empty chunks, and empty/no-op updates instead of silently accepting them
• require *** End of File chunks to match at EOF rather than falling back to an earlier match
• include patch line, operation, path, and underlying filesystem or matching context in failures
• include existing destination content in structured diffs when an add or move overwrites a file

Further follow-up

These differences remain open rather than being deferred by a product decision:

• determine whether move execution should gain coordinated source verification and two-target mutation safety
• improve exact or explicitly inexact committed-change accounting after partial failures
• evaluate the remaining parser/application performance differences
• decide whether any Codex success-output grouping is desirable beyond OpenCode’s current operation-ordered plain output

Current scope decisions

• omit Lark/freeform tooling until the V2 tool infrastructure supports it
• do not copy Codex permission behavior because OpenCode uses a different permission model
• leave invalid UTF-8 behavior unchanged for now while preserving OpenCode BOM handling
• omit Codex-specific streamed previews, multiple-environment routing, shell interception, and standalone executable integration

Review

• independently compared against the Codex patch parser and tool implementation
• ported Codex coverage for implicit initial update chunks and CRLF line-ending behavior