[information-architecture] Workflows nav links to missing documentation pages

`mkdocs.yml` includes multiple Workflows navigation entries that point to Markdown files that do not exist under `docs/`, creating dead-end routes and breaking discoverability.

Concrete example:
- `mkdocs.yml:36` maps **Duplicate Issue Detector** to `workflows/gh-agent-workflows/duplicate-issue-detector.md`
- File is missing: `docs/workflows/gh-agent-workflows/duplicate-issue-detector.md`

Additional missing targets are present in the same nav section (`mkdocs.yml:38-80`), resulting in 27 missing nav destinations total.

User impact:
- Users cannot navigate to advertised workflow docs from primary nav.
- Major feature areas appear available but are unreachable, causing confusion and trust loss.

Suggested fix:
- Either add the missing docs pages or remove/hide these entries from nav until pages exist.
- Add CI validation that every `.md` nav target in `mkdocs.yml` exists.

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