Release Ceremony Refinement: Adopt epic-XXX Branches and Per-Track Next Release Issues

Stub Epic for remaining release ceremony changes discussed in #459 closing.

Goal (high-level): Evolve the PLATE branch model and release negotiation process to better support per-Epic isolation and explicit semver track negotiation:

• Use dedicated epic-XXX branches (e.g., epic-459) owned by each Epic for its integration work. Feature/Bug PRs for the Epic target this branch (after receiving Major/Minor/Patch label). Epic-close PR from epic-XXX → the appropriate track branch (release, release-major, etc.).
• Maintain separate standing "Next Major Release", "Next Minor Release", and "Next Patch Release" issues (in addition to or evolving the single "Next Release" #449). Epics declare track via label and link to the matching Next X Release issue via the Development sidebar for negotiation/targeting/on-hold visibility.
• At Epic planning/closing (including via plate_plan_epic), determine/confirm semver track, explicitly update the corresponding Next X Release issue to reference the Epic.
• Update gh plate release status to surface per-track pending Epics, fragments, and links.
• Update AGENTS.md (Branch Model, Epic-close, Release ceremony sections), docs/design/release-ceremony-refinement.md, bootstrap, and related docs/scripts to document and support this.
• Handle legacy single-release transition and multi-track.
• Ensure fragments for any process changes; maintain one standing Next Release during transition if needed.

Current status: Stub. Details, exact scope, child issues (e.g., updates to plate_plan_epic, release status, docs, bootstrap), acceptance criteria, and design to be developed as direct follow-on to #459 (default persona work) and prior release design.

See discussion in #459 (and Quiet Agents #456, Scriptify #462) for context. Use status:stub and need:refinement as appropriate.

Start with the discussion in #459 and the prior release design, then inspect AGENTS.md, docs/design/release-ceremony-refinement.md, bootstrap, plate_plan_epic, and gh plate release status. The scope, child issues, acceptance criteria, and design are explicitly still to be developed; done means those details and the legacy-transition and multi-track process are defined and documented.