Recognize a Compose Mapping Key Containing # Before a Block Scalar in the Prose Gate #3717

Raised by the review of promotion #3715 on the change #3712 made, listed without a thread.

The prose gate's Compose reader now treats a key line ending in a YAML block-scalar indicator (| or >) as opening opaque text. Copilot reports that a valid mapping key containing # is not recognized as such a key, so the block scalar after it is not skipped. A # is a comment only after whitespace, so x-a#b: | is a valid key. Constructed shape:

services:
  app:
    x-a#b: |
      volumes:
        - cache:/home/zed/data

If the reader strips from the first # before matching the key, the indicator is lost and the mount-shaped text is read as a volume entry. This is to be confirmed by running the gate on that case before any fix.

Backlog burndown claim. Branch: feature/burndown-compose-hash-key. Predicted file set: .github/actions/prose-gate/prose_lint.py, tests/test_prose_lint.py.