Context Decomposition for Invocation and Workspace Scope

Tracking ticket for RFD 113.

Implementation plan

Introduce the two types and migrate plugin dispatch: Adds CliCtx and WorkspaceCtx with Ctx as a shell holding both. Resolves delegated-turn configuration through ConfigPipeline per checkout, deletes Ctx::swap_config, keys MCP server instances by server/configuration/spawn directory with per-turn leasing (deleting configure_active_mcp_servers, set_servers, McpServerScope), warns instead of silently skipping misdirected background tasks, and migrates run_external, run_plugin, message_loop, and plugin request handlers to the two halves. Mergeable independently; unblocks multi-workspace addressing.
Migrate remaining commands off the shell: Moves every other command from the Ctx shell to CliCtx/WorkspaceCtx, one group at a time, then deletes the shell. Depends on Phase 1; each command group is independently mergeable.
Record the inventory and enact the placements it forces: Documents the application/frontend capability split under docs/architecture/, consolidates query-draft handling into the storage layer (one path resolution shared by CLI and FFI, preserving both conflict policies), and removes cfg from PluginToHost::Query and list_configs from the protocol surface (still deserializable, but answered with a correlated error so no turn starts under a dropped config selection). Depends on Phase 1; each item is independently mergeable and is its own behavior-change review.