feat(cli): workspace management verbs + docs (MCP stays read-only)

[dvcdsys]

Summary

Adds workspace management to the cix CLI. Previously cix ws was
read-only (list / describe / repos / search) and creating a workspace
or attaching projects was dashboard-only. The server already exposed all the
mutation endpoints — this wires the CLI to them and documents the surface.

MCP is intentionally left read-only (no new cix_* tools); the docs just
clarify that management lives in the CLI/dashboard.

New CLI verbs (name-first grammar)

cix ws create "<name>" [--description "…"]
cix ws "<name>" add <project…>            # link an already-indexed project
cix ws "<name>" remove <project…>         # unlink
cix ws "<name>" rename "<new-name>"
cix ws "<name>" update [--name "<new>"] [--description "…"]
cix ws "<name>" delete [-y]               # prompts on a TTY; -y to skip

A <project> is any already-indexed project addressed by absolute path,
host_path (github.com/owner/repo@main), or 16-hex path_hash; no arg
defaults to the cwd. add links an existing index — it does not clone;
cloning a new GitHub repo into a workspace stays a dashboard/git-repos
operation.

Notable implementation details

• client (internal/client/workspace.go): CreateWorkspace,
  UpdateWorkspace, DeleteWorkspace, LinkProjectToWorkspace,
  UnlinkProjectFromWorkspace.
• Local-project resolution: a local project's server host_path is the
  namespaced key local:{machine_id}:{path} (bare path is in
  container_path), so add <abs-path> resolves by re-deriving the
  path_hash (EncodeProjectPath), not by a host_path string compare. This
  was a real bug caught in end-to-end testing.
• Safety: delete prompts on a TTY and refuses on a pipe without --yes
  (never deletes without explicit consent). delete removes the workspace +
  its membership links only — the underlying projects are untouched.
• SilenceUsage/SilenceErrors on the ws command so errors read as one
  clean line.

Docs / skills (2nd commit)

• doc/CLI_REFERENCE.md, skills/cix/SKILL.md (+ plugin copy): Workspaces
  "Manage" block.
• workspaces.md, doc/WORKSPACES.md: CLI link-existing flow, kept distinct
  from the dashboard clone-new flow.
• skills/cix-workspace/SKILL.md: prerequisite note on creating/populating a
  workspace.
• doc/COWORK_MCP.md, plugins/cix-cowork/skills/cix-workspace/SKILL.md:
  clarify the MCP surface is read-only.
• Canonical skills mirrored into plugins/cix via sync-skills.sh.

Testing

• go build / go vet / go test ./... green; 12 new tests in
  cli/cmd/workspace_test.go.
• End-to-end against a live server: create → add(abs path) → dup(409) →
  unknown → list → describe → rename → update → remove → delete → 404.
• sync-skills.sh --check clean; plugin bats suite 15/15.

Server-side: no changes (endpoints pre-existed). No openapi/AUTH_REVIEW
changes needed.

🤖 Generated with Claude Code