docs(e2e): exact ANTHROPIC_API_KEY setup to enable the 4 LLM specs; correct the gate rationale

Investigated whether the four LLM-gated specs (chat rename, chat delete,
project-assistant create+submit, critical path) could run keyless with the
demo model they select. They cannot on this codebase: no keyless model
exists here - ModelToggle's MODELS are all Anthropic/Google/OpenAI entries
gated on a configured provider key (env or user-stored,
backend/src/lib/userApiKeys.ts), ChatInput.handleSubmit hard-blocks a send
for an unavailable model, and the backend model set
(backend/src/lib/llm/models.ts ALL_MODELS) has no demo id either. The
'Demo (no key needed)' model the specs target lives only in the amal66
fork. So the blanket test.skip(!hasLlmKey, ...) guard stays as-is - the
evidence does not support narrowing it.

What this change does instead:
- docs/e2e-ci.md: new 'Enable the LLM specs' section with the exact UI
  path and gh CLI command for the ANTHROPIC_API_KEY repository secret, the
  fork-PR secrets caveat (use a maintainer branch or workflow_dispatch to
  exercise the specs), expected per-run cost, and how to confirm in the
  Actions log that the specs ran (27 passed, no skipped line) rather than
  skipped (4 skipped / 23 passed). Also documents the known gap: until the
  specs' selectDemoModel helper is pointed at a Claude model (or the demo
  provider is upstreamed), the secret unskips the specs but they fail at
  model selection.
- e2e/llm.ts: rewrite the doc comment with the precise gate rationale
  (no keyless model in this repo; title-generation 500s are best-effort
  and not the reason) and pointers to the setup docs.
- .github/workflows/e2e.yml: comment-only updates aligning the skip
  narrative (23 keyless specs, not ~20/~23) and pointing at the new docs
  section. No behavioral change anywhere.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>