docs(spec): add spec 057 for in-proxy profiles (#55)

Captures the design discussion from issue #55 (Dumbris's
"In-Proxy Profiles + Permanent URLs" plan plus the scope
trade-offs from the follow-up comments) into a spec doc so the
implementation can be reviewed in two stages (spec first, code
after), matching the pattern from spec 056 / PR #525.

Scope:
- profiles config + /mcp/p/{slug} pinned URL
- filter at retrieve_tools / call_tool_* hooks
- composes with agent-token AllowedServers (Spec 028) and
  per-user visibility (Spec 029) by intersection
- existing per-server enabled_tools / disabled_tools reused
  for tool-level filtering inside a profile

Out of scope (deferred to follow-up specs/PRs):
- active profile switching (state ownership is an open
  question for the maintainer)
- tray UI / set_profile MCP tool
- index-level profile field
- mixed server / server:tool list per profile

Refs: #55, #333

Author: Ylsssq926