docs: add PR description and dependency link requirements to contributing guidelines

Added explicit requirement that every PR must include a detailed description
with a summary, closing keywords for linked issues, and a Forgejo dependency
link. Updated the merge checklist with PR description and dependency link
verification items. Reworked PR Process item 1 to be comprehensive about
both the description body and the issue linkage.

Refs: #408

CONTRIBUTING.md

@@ -226,10 +226,21 @@ ISSUES CLOSED: #280
 
 Every Pull Request must meet the following requirements before it is submitted for review:
 
-1. **Reference at least one issue.** Every PR must be linked to at least one issue. Include
-   the issue reference in the PR description using a keyword that Forgejo recognizes (e.g.,
-   `Closes #45`, `Fixes #45`, or `Refs #45`). If your change is not associated with an
-   existing issue, create one first — see [Creating Issues](#creating-issues).
+1. **Provide a detailed description.** Every PR must include a clear, descriptive body that
+    explains the purpose of the change, summarizes what was done and why, and provides enough
+    context for a reviewer to understand the PR without reading every line of code. At a
+    minimum, the description must contain:
+    - A **summary** of the changes and the motivation behind them.
+    - An **issue reference** using a closing keyword that Forgejo recognizes (e.g.,
+      `Closes #45`, `Fixes #45`) so that the linked issue is automatically closed when the PR
+      is merged. If the PR addresses multiple issues, include a closing keyword for each.
+    - A **dependency link**: in addition to the textual reference, add the linked issue as a
+      Forgejo dependency on the PR so that the relationship is visible in the issue tracker's
+      dependency graph.
+
+    If your change is not associated with an existing issue, create one first — see
+    [Creating Issues](#creating-issues). PRs submitted without a description or without an
+    issue reference will not be reviewed.
 2. **One Epic scope per PR.** Each PR must be associated with a single Epic. Do not combine
    work from multiple unrelated Epics in one PR. If your changes span multiple Epics, split
    them into separate PRs.
@@ -327,6 +338,9 @@ All CI pipeline checks must pass. This includes, but is not limited to:
 
 Before a PR is merged, confirm the following:
 
+- [ ] PR description is detailed, explains the change, and includes closing keywords for all
+      linked issues (e.g., `Closes #45`)
+- [ ] Linked issues are added as Forgejo dependencies on the PR
 - [ ] All CI checks pass (tests, linting, type checking, security, coverage)
 - [ ] Test coverage meets or exceeds the project-defined threshold
 - [ ] At least two approving reviews from non-author contributors