docs(planning): add plan for dynamic MCP toggle meta tools

5-phase plan layered on top of PR #151's `mcpmux_*` meta-tool surface:
adds `mcpmux_list_servers`, `mcpmux_enable_server`, `mcpmux_disable_server`
with a session-scoped default (in-memory `SessionOverrideRegistry`) and a
workspace-scope opt-in that reuses the existing `WorkspaceBinding` write
path. Resolver stays untouched; composition lives in `FeatureService`.

Closes the gap between PR #151's persistent bindings and the ephemeral
"LLM-driven, minimum-context-default" workflow from the jsg-tech-check
homelab plan.

Signed-off-by: crimsonsunset <jsangio1@gmail.com>

Author: crimsonsunset