Improve Github Handle detection in legacy project-managed maintainer files

[RobertKielty]

Background

We currently parse maintainer refs using a set of regexes (e.g., @handle, github.com/handle, YAML list items). This fails on many legacy maintainer files that present GitHub handles inside markdown tables, especially when the handles are not prefixed with @.

Example (current failing case)

  | Maintainer | GitHub ID | Affiliation |
  |---|---|---|
  | Ann Maintainer | amaintainer | ACME INC. |

Today amaintainer is not detected because it appears only in a table column.

Impact

Missing handles means:

• NOT PRESENT ON CNCF DATABASE lists are incomplete on the UI
• Add Maintainer buttons don’t appear for table‑only handles
• UAT testers see discrepancies between project files and maintainer‑d

Expected

Handles in columns labeled GitHub ID / GitHub Handle / GitHub Username should be detected as GitHub accounts, even without @ prefixes.

Proposed approach

• Add markdown table parsing to the maintainer‑ref parser
• Detect headers like GitHub ID, GitHub Handle, GitHub Username
• Extract values from that column and validate as GitHub handles
• Use this as a deterministic rule before any LLM fallback

Acceptance criteria

• Table‑only handles are detected
• No regression in existing @handle / github.com/handle / YAML parsing
• Unit test covers the example above and passes

Notes
We can implement this in a shared parsing utility to keep UI + tooling consistent.