feat(context): toolsets, tool annotations, response-size guard (Stage 1) (#10)

Reduce MCP client context cost and add safety metadata:

- PLANE_TOOLSETS env selects tool groups (core/comments/pages/types/
  planning/admin/pql or all). Default all (backward compatible); core is
  ~64 tools vs ~142. New list_toolsets diagnostic.
- Central annotations.py derives readOnlyHint/destructiveHint/idempotentHint/
  openWorldHint for every tool from its name prefix.
- Read-only surface now keys off readOnlyHint (single source of truth) instead
  of a separate name-prefix list.
- PLANE_MAX_RESPONSE_KB caps list_work_items/list_archived_work_items payloads;
  over the limit returns an actionable error instead of blowing up client context.

Tests: test_toolsets.py + extended test_readonly_surface.py. 103 passing.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>