Throughstone v1.2.0 — discoverability & docs hygiene

A discoverability & docs-hygiene release: it indexes the runbook and registry folders, adds
the secrets-rotation runbook the operate-time set was missing, makes the session set
flexible for added/conditional sessions, and closes plain-language gaps the method's own L1/L2
standard exposed.

New operate-time runbook

• Secrets rotation (runbooks/secrets-rotation.md): scheduled rotation (inventory, cadence,
  no-downtime overlap, verify-then-revoke) and a revoke-first response to a suspected leak
  that hands off to the incident runbook. Operationalizes the threat-model §5 posture, mirroring
  how the dependency runbook operationalizes §6.

Discoverability & docs hygiene

• README indexes for runbooks/ (all five — purpose, when each fires + trigger phrase,
  governing section; STEP-shaped vs. operational) and registries/ (machine-readable state,
  pointing at repos.yml's own header rather than duplicating the schema); docs-hub rows now link
  both indexes.
• Conditional-session naming is shown, not just described: the by-name → file mapping inline
  in METHOD §4, and a copyable lettered-row example in the STEP-index seed.
• Plain-language glosses for jargon flagged against the method's own L1/L2 standard:
  API / OpenAPI / GraphQL / protobuf (session 1.3) and the RPO/RTO acronyms (session 1.8).

Flexible session set

• Session numbering no longer hardcodes the current set: the conditional-doc rule and
  status.sh review-detection adapt to added sessions; dependency-bearing sessions
  (1.5 / 1.7 / 1.8 / 1.10 / 1.11) read relevant conditional docs when present.
• Glossary session (1.12) harvests terms from every architecture doc (including conditional
  docs 13+), not a fixed range.
• METHOD §4 "Adding a session" recipe — conditional (zero-touch) vs. standard (renumber the
  cross-cutting review) wire-in checklist.