README: pi 'What gets installed' section catchup

Was stale:
- Claimed 6 pi-extensions (actually 7 — mcp-loader was added in
  pi-extensions 141bf64 / 7eec49b / 37cc49e but the count was never
  propagated here).
- No mention of mcp-loader's dual-transport (local stdio + remote
  streamable-HTTP per MCP spec 2025-03-26) or the /mcp slash command.
- Mempalace bridge bullet didn't note that it coexists with mcp-loader
  rather than being replaced by it (don't list mempalace in mcp block).
- No explicit 'no MCP servers baked in' line, leaving readers to
  guess whether searxng/context7 ship by default.

Each extension now gets a one-line description; mcp-loader gets a
paragraph covering its capabilities and a link to the pi-extensions
AGENTS for transport detail. Added an opt-in note for MCP servers.

CHANGELOG.md

@@ -23,6 +23,7 @@ Docs-only (the DOCKER_HUB.md change can be patched live to Hub without a CI rebu
 
 - **Hub doc rewrite:** `DOCKER_HUB.md` is now generated from a hand-maintained `HUB_TEMPLATE` constant in `scripts/generate-dockerhub-md.py` instead of a section-by-section transformation of `README.md`. Drops from 24 997 bytes (3 byte headroom) to 5 551 bytes (~78% headroom). The old derive-from-README mechanism (`SECTION_RULES`, `TRIM_SUBSECTIONS`, `REPLACEMENTS`, `split_sections`, `trim_subsections`) is gone — README and Hub doc are now independent surfaces. Hub copy stays slim and links out to the gitea README for full depth (build args, multi-user setup, AWS Bedrock walkthrough, MemPalace deep-dive, language-specific dev sections). Trade-off: image-variants table and quick-start flow are now coupled to `HUB_TEMPLATE` and need a manual edit when they change — explicit and local rather than spread across rules.
 - **AGENTS.md:** "Documentation coupling on release" rule updated — README edits no longer require regenerating DOCKER_HUB.md. Release-day checklist tightened.
+- **README pi section:** "What gets installed" sub-section updated to reflect the actual shipped state. Was stale: claimed 6 pi-extensions (actually 7 — mcp-loader was added in pi-extensions commit 141bf64 / 7eec49b / 37cc49e but never propagated here). Each extension now has a one-line description; mcp-loader gets a paragraph covering its dual-transport (local stdio + remote streamable-HTTP per MCP spec 2025-03-26) capability and the `/mcp` slash command. Clarified that the mempalace bridge is a separate MCP entry point that coexists with mcp-loader rather than being replaced by it. Added an explicit note that no MCP servers are baked in beyond mempalace — the loader is opt-in via settings.json edits.
 
 ## v1.14.41b — 2026-05-08