docs(planning): tool-level session pin meta tool

Planning doc for `mcpmux_pin_this_session` + `mcpmux_clear_session_pin`
- ephemeral session-scope tool-list pin with optional workspace persistence.
Revisits the deferred tool-level-granularity decision from
dynamic-mcp-toggle-meta-tools in light of the 240-tool context bloat
exposed by the Google Workspace clone case.

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

Author: crimsonsunset