[feature]: support cycles in Compose YAML (currently dropped on plane clone)

Context for this and four related tickets: https://github.com/orgs/makeplane/discussions/8999

I have searched existing issues; no existing report covers this specific gap.

Summary

Compose 0.2.0 doesn't represent cycles -- neither in schema/ nor as a field on work items. plane clone drops cycle definitions (including start / end dates) and per-item cycle memberships silently. Particularly costly for projects with overlapping cycle windows: that's a Plane differentiator that's completely invisible to Compose.

Proposed shape:

# schema/cycles.yaml
- name: Diligence
  start: 2026-05-04
  end: 2026-06-01
- name: Pre-Close
  start: 2026-06-08
  end: 2026-06-29

# work item
- id: PROJ-1
  title: ...
  cycle: Diligence

Bonus: an optional date-shift at clone time (plane clone <uuid> --shift-dates +6mo or similar) would unlock template reuse for projects that recur on a calendar -- release planning, deal pipelines, marketing campaigns, etc. Without it, every clone of a date-bearing template requires manual fixup of every cycle window and every item's start / due date.

Why should this be worked on?

Cycles are the time dimension of a Plane project. Without them, Compose can only describe a static structure, not a phased one. For any project that uses cycles for sprint or phase management -- which is most non-trivial projects in Plane -- the current behavior means the time-phasing layer can't be source-controlled, can't be PR-reviewed when it changes, and can't be templated for reuse.

To reproduce the gap: clone any Plane project that uses cycles (e.g. via plane clone <uuid> --workspace <slug>), then inspect schema/ and .plane/remote/items.yaml. Cycles and per-item cycle membership are absent.

Combined with the modules gap (#9003) and the relations gap (tracked at #6236), supporting cycles in YAML is one of the three structural primitives needed for the projects-as-code thesis to hold up for non-flat projects. Related tickets in the same batch: #9000 (missing CLI commands), #9001 (push is create-only), #9002 (version drift). The discussion thread linked above frames all five as a coherent set.

Start by reproducing the issue with plane clone <uuid> --workspace <slug>, then inspect schema/ and .plane/remote/items.yaml. Trace how cycle definitions and per-item cycle memberships are represented or dropped. Done means cycle names, start and end dates, and item memberships survive clone and are represented in Compose YAML.