docs(personas): document intent of pre-staged personas

The audit flagged DevOpsOrchestrator, AIEngineer, and Maintainer as
orphans — defined in personas.yaml but never referenced as `persona:`
on any prompt. The user chose to keep them and document the intent
rather than remove them.

This commit adds:

- A top-of-file comment block explaining the pre-staging convention:
  the three personas reserve canonical names and responsibilities
  for post-v0.5.0 phases (CI, MCP/agent runtime, long-term repo
  maintenance — all in ROADMAP §9 as out of scope for v0.5.0).
  Pre-allocating them avoids silently breaking consuming projects
  that already adopt these IDs in their own specs.
- Per-persona descriptions extended with a one-line note clarifying
  the pre-staged status and pointing at the future phase that will
  activate them.
- Maintainer's note also references the worklog co-sign role, since
  the persona does appear there even though no prompt declares it as
  `persona:`.
- Persona list `version` bumped 2 → 3 to reflect the substantive
  description change.

See ROADMAP §7.4 (added in the previous commit) for the deprecation
policy that applies if any pre-staged persona is ever retired.

Verified: personas.yaml continues to validate against
persona.schema.json.

Part of Phase 6 (v0.5.1 polish) — surfaced by the post-v0.5.0 audit.