Default turn-doc diffs to @pierre/diffs

Summary

Updated repository turn-documentation rules to prefer @pierre/diffs for diff snippets, added a documented fallback path, and included the package/lock updates needed for consistent local usage.

Changes Made

• Edited AGENTS.md to add a default diff-format policy for turn docs.
• Updated the minor-update template bullet for Code diffs with explicit @pierre/diffs default plus fallback wording.
• Updated required section Relevant Diff Snippets with the same default-and-fallback wording.
• Added @pierre/diffs to root package.json dependencies and synced bun.lock.

Context

The existing guidance required a diff section but did not explicitly standardize on a single rendering tool. This change aligns turn-doc expectations around one default tool while preserving an escape hatch when tooling is unavailable.

Important Implementation Details

• The policy is intentionally a preferred default, not a hard requirement.
• Fallback usage is constrained to real tool/blocking errors and must be labeled with a reason.
• No runtime application behavior was changed; this is workflow/documentation and dependency-surface work.

Relevant Diff Snippets

@pierre/diffs is now the repository default for this section.

--- AGENTS.md
+++ AGENTS.md
@@
+For diff content in turn documentation (including "Code diffs" and "Relevant Diff Snippets"), use `@pierre/diffs` output by default.
@@
-- **Code diffs**
+- **Code diffs** (use `@pierre/diffs` output by default; if unavailable, include a clearly labeled plain diff/code block and note why)
@@
-5. **Relevant Diff Snippets**
+5. **Relevant Diff Snippets** (render with `@pierre/diffs` output by default; if unavailable, include a clearly labeled plain diff/code block and note why)

--- package.json
+++ package.json
@@
+  "dependencies": {
+    "@pierre/diffs": "^1.2.2"
+  }

Expected Impact for End-Users

• Future turn docs should have more consistent and readable diff presentation.
• Contributors have clearer guidance for fallback behavior when the preferred renderer cannot be used.

Validation

• Verified policy text appears in all required AGENTS locations for diff guidance.
• Ran bun install --frozen-lockfile and confirmed lockfile consistency with no changes.
• Confirmed repository is clean and branch is up to date after push.

Issues, Limitations, and Mitigations

• This change standardizes policy but does not retroactively update old turn docs.
• Actual visual rendering still depends on environment/tool availability; fallback instructions mitigate this.

Follow-up Work

• Optionally add a tiny helper script/example to generate @pierre/diffs HTML snippets directly for turn docs.