docs: guide for granting headless environments read access to private docs

[RubenGlez]

A real session surfaced the gap: a remote coding agent working in a repo
that uses doctier could not read any private doc (by design), but there
was no documented path to deliberately grant such an environment access —
the pieces (grant, DOCTIER_SSH_KEY, unlock/cat) had to be
reverse-engineered from scattered README fragments, and the CI recipes
only cover the no-key paths.

• Add docs/agents.md: generate a dedicated passphrase-less key, have an
  existing recipient grant it (the environment cannot self-serve — a key
  added after encryption cannot decrypt earlier blobs), provision the
  private half as a secret via DOCTIER_SSH_KEY, then unlock/cat; plus
  the revoke flow and an explicit trade-off callout (dedicated key per
  environment, never a personal one).
• Link the guide from the README's "Joining a repo" section (including
  the "agent pasted me an AGE ENCRYPTED FILE block" moment) and from the
  CI paragraph.
• Point the failure UX at the guide: keyless unlock/cat now explain
  in one line how access is granted and why self-adding a key won't
  work; the wrong-key paths (not a recipient) do the same.
• Align unlock -h on $DOCTIER_SSH_KEY, the variable the README
  documents (both variables remain honored).

Co-Authored-By: Claude Fable 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01MdC78SeCdRXuinYtMS4UEC