MCP via pi's built-in support: mcp-loader retired upstream; skill floor, docs, sanity check

pi-extensions 750b2a8 retired mcp-loader.ts in favour of builtin:mcp (an
extension registering /mcp replaced the built-in and printed a warning on
every start). This repo has no pin to move, since PI_EXTENSIONS_REF and
MEMPALACE_TOOLKIT_REF float on main, so this commit carries what the next
build changes:

- CHANGELOG Unreleased: component table (pi-extensions a88fa99 -> 750b2a8,
  mempalace-toolkit 975ab92 -> a30af61), the switch and its measurements, the
  stale-link pruning, the new sanity check, and the floor refresh.
  check-doc-drift.sh rc=0. A two-sided check: with 750b2a8 blanked out of the
  CHANGELOG it fails rc=1, naming pi-extensions a88fa99 -> 750b2a8.
- rootfs pi-extensions skill floor refreshed to 750b2a8. check-skill-floor.sh
  was rc=1 before and is rc=0 after. Base hash changes, so the next tag
  rebuilds the base.
- recreate-sanity-check.sh: "no dangling extension links" (WARN). The
  existing count includes dangling links because ls lists them. Tested
  two-sided in a temp HOME (PASS clean; WARN naming mcp-loader.ts and x.ts.off).
  On tor-ms22 v1.11.0: rc=0, 36 pass / 3 warn / 0 fail (was 35/3/0).
- DOCKER_HUB.md: pi-extensions list was three releases stale; it now lists the
  10 that ship, plus an MCP line. Dockerfile.variant comment updated.
- lint-shell.sh: 19 files clean at severity error.

Dockerfile.variant

@@ -212,9 +212,11 @@ ARG USER_NAME=developer
 # `exposure` is `codemode`, i.e. such tools are "neither declared to the model
 # nor listed" and must be found with `searchTools()`. That would gut the
 # MemPalace protocol if MemPalace were a builtin-MCP server. It is not: both
-# mempalace.ts and mcp-loader.ts run their OWN MCP client and surface tools via
+# mempalace.ts runs its OWN MCP client and surfaces tools via
 # `pi.registerTool()`, which is why they are named `mempalace_search` and not
-# `mcp__mempalace__search`. Confirmed on a second route — settings.json has
+# `mcp__mempalace__search`. (So did pi-extensions' mcp-loader.ts until it was
+# retired after v1.11.0 in favour of builtin:mcp. Servers in ~/.pi/agent/mcp.json
+# ARE exposure-governed: they need `"exposure": "direct"` to be declared.) Confirmed on a second route — settings.json has
 # neither an `mcpServers` block (builtin, exposure-governed) nor an `mcp` block.
 # Extension-registered tools are declared like built-ins, so `exposure` cannot
 # reach them.