test: fix two pre-existing just test failures

1. helix-e3c7d0e4 (Epic: per-runtime integration tests) was missing a
   `<context-digest>` block in its description. The bead is still open,
   but all 5 child runtime beads (INT-CC/INT-CX/INT-CP/INT-GN/INT-DD)
   are closed. Add a digest summarizing the epic's governing context
   (principles, concerns, practices, governing artifacts) so
   `tests/validate-context-digests.sh` passes.

2. tests/validate-demo-fixtures.sh was checking a SHA-256 coherence
   between docs/demos/helix-concerns/demo.sh's REVIEW_PROMPT heredoc
   and an agent-dictionary/<hash>.json fixture. Commit 9e7f734
   ("demos(concerns): add drift fixture + README; drop legacy CLI
   artifacts") deleted demo.sh and the agent-dictionary entirely in
   favor of a recorded session.jsonl. The test has been failing on
   main since that commit and its premise (prompt-hash fixture
   coherence) no longer applies. Remove the test + justfile wiring.

`just test` is now fully green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: easel