skein CLI over the plugin RPC + server API (P1.8) #131

Parent: Phase 1a item P1.8. Slots after adopt-upstream-acp-and-service's verification
chain (it documents the same surfaces), but scaffoldable in parallel.

Why

The opencode loop|agents|beads subcommands die with the fork. Charter §17 requires the
platform to be diagnosable without the dashboard: skein status, skein projects,
skein agents, skein work ready|running|blocked, skein fleet, skein trace <execution-id>, skein doctor. A standalone binary over the plugins' RPC and the
opencode server API replaces the subcommands and becomes the control plane's
user-facing surface later.

What Changes

1. skein-cli package (language decision in design.md — recommendation: Go, per
   charter §9.3's preference and the control plane's language; the RPC/server client is
   thin) driving the plugin RPC surfaces (locations per P1.2 trap 5/7 discipline) and
   the opencode server API.
2. Implement the §17 command set with the correlation-ID plumbing available to
   skein trace once skein-observability lands; until then trace renders what IDs
   exist (session/execution) and says explicitly what is missing — never fabricates.
3. skein doctor includes the planning-freshness check (docs/skein-2 copy vs loose
   companions, K15) and the plugin health probes (one effect in tree, resolution
   probe) to make CI's gates visible locally.
4. Replaces the opencode loop|agents|beads subcommands referenced in
   port-beads-plugin's interim parity; that interim path is retired with evidence
   that the CLI reads/writes the same records.

Non-goals

No dashboard UI (mission-control). No new authority — the CLI acts with the invoking
user's grants, never as the lead (charter §14.1).

Risks

• Verb drift from specsync/openspec CLI conventions — pin command shapes to the §17
  list and keep verbs stable.

Impact

New binary replacing the inherited subcommands; diagnostic surface for every later
phase; sentinel for observability IDs.