yaml: support block scalars (key: | and key: >) for multi-line run/envVerified

cave's YAML parser returned the literal "|" for any `run: |` block, so every
multi-line step (ubiquitous in GitHub-Actions-style workflows) ran "bash -c |"
and failed with a syntax error. Add block-scalar parsing: a `key: |`/`key: >`
header swallows its more-indented body verbatim (literal or folded, with -/+
chomping), with comments preserved inside the body so shell `#` comments in a
`run: |` survive. Internally lines became (indent content blockval) triples.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>