Add CITATION.cff to bump-version skill

.claude/commands/bump-version.md

@@ -25,6 +25,7 @@ Files that need updating:
 | `rust/Cargo.toml` | `version = "X.Y.Z"` | ~3 |
 | `CHANGELOG.md` | Section header + comparison link | Top + bottom |
 | `diff_diff/guides/llms-full.txt` | `- Version: X.Y.Z` | ~5 |
+| `CITATION.cff` | `version: "X.Y.Z"` + `date-released: "YYYY-MM-DD"` | ~10, ~11 |
 
 ## Instructions
 
@@ -37,11 +38,19 @@ Files that need updating:
    - Read `diff_diff/__init__.py` and extract the current `__version__` value
    - Store as `OLD_VERSION` for comparison link generation
 
-3. **Check CHANGELOG entry**:
-   - Search `CHANGELOG.md` for `## [NEW_VERSION]` section header
-   - If found: Verify it has content (at least one `### Added/Changed/Fixed` subsection with bullet points)
-   - If not found or empty: Generate entry from git commits (step 4)
-   - If found with content: Skip to step 5
+3. **Check CHANGELOG entry and resolve `RELEASE_DATE`**:
+   - Search `CHANGELOG.md` for `## [NEW_VERSION]` section header.
+   - If found with content (at least one `### Added/Changed/Fixed` subsection with
+     bullet points):
+     - **Parse the existing header date** (e.g., `## [3.1.3] - 2026-04-19` → `2026-04-19`).
+       Store as `RELEASE_DATE` and skip to step 5.
+     - If the header has no date (malformed), abort with: `Error: CHANGELOG header for
+       [NEW_VERSION] is missing a date. Fix the header before re-running.`
+   - If not found or empty: Set `RELEASE_DATE` to today's date in `YYYY-MM-DD` format,
+     then continue to step 4.
+
+   `RELEASE_DATE` is the single source of truth for the release date across every file
+   touched in this bump. Do not recompute it downstream.
 
 4. **Generate CHANGELOG from git** (only if needed):
    - Run: `git log v{OLD_VERSION}..HEAD --oneline`
@@ -50,7 +59,7 @@ Files that need updating:
      - **Added**: commits containing "add", "new", "implement", "introduce", "create"
      - **Changed**: commits containing "update", "change", "improve", "optimize", "refactor", "enhance"
      - **Fixed**: commits containing "fix", "bug", "correct", "repair", "resolve"
-   - Get today's date in YYYY-MM-DD format
+   - Use the `RELEASE_DATE` resolved in step 3 for the header.
    - Create CHANGELOG entry in this format:
      ```markdown
      ## [X.Y.Z] - YYYY-MM-DD
@@ -83,6 +92,13 @@ Files that need updating:
    - `diff_diff/guides/llms-full.txt`:
      Replace `- Version: OLD_VERSION` with `- Version: NEW_VERSION`
 
+   - `CITATION.cff`:
+     Replace `version: "OLD_VERSION"` with `version: "NEW_VERSION"`.
+     Also update `date-released: "OLD_DATE"` to `date-released: "{RELEASE_DATE}"`
+     using the `RELEASE_DATE` resolved in step 3. Both fields are quoted strings;
+     preserve the quoting style. `RELEASE_DATE` must match the CHANGELOG header
+     date; never substitute a freshly computed "today" value here.
+
 6. **Update CHANGELOG comparison links**:
    - Run `git remote get-url origin` to determine the repository's GitHub URL
      (strip `.git` suffix, convert SSH format to HTTPS if needed)
@@ -102,6 +118,7 @@ Files that need updating:
    - pyproject.toml: version = "NEW_VERSION"
    - rust/Cargo.toml: version = "NEW_VERSION"
    - diff_diff/guides/llms-full.txt: Version: NEW_VERSION
+   - CITATION.cff: version: NEW_VERSION, date-released: YYYY-MM-DD
    - CHANGELOG.md: Added/verified [NEW_VERSION] entry
 
    Next steps:
@@ -117,3 +134,10 @@ Files that need updating:
 - If CHANGELOG already has the target version entry with content, it will not be overwritten
 - Commit messages are cleaned up (prefixes like "feat:", "fix:" are removed) for CHANGELOG
 - The comparison link format uses `v` prefix for tags (e.g., `v2.2.0`)
+- `CITATION.cff` `date-released` and the `CHANGELOG.md` section header share a single
+  `RELEASE_DATE` resolved in step 3: if the CHANGELOG entry was pre-populated, its
+  existing header date wins (so pre-written changelog drafts don't silently drift
+  from the CITATION date); otherwise today's date is used for both. If the release
+  is cut on a different day than the bump, update both surfaces manually — drift
+  causes auto-citation tools (Zenodo, GitHub's "cite this repository", reference
+  managers) to report stale metadata.