[Ticket]: Python-backed MCP batch — mem0, MCP-Mathematics, pdfmux (packaging decision)

Part of #549

## Type
Feature

## Summary
Add the **Python-backed MCP batch** deferred from ADR-0071: **mem0**, **MCP-Mathematics** (both `uvx`), and **pdfmux** (npm wrapper that still needs `pip install pdfmux` + an LLM key). Requires a packaging decision since these are not bare-npx.

## Intent
Deliver PDF handling (explicitly requested — users have PDFs in projects), math, and long-term memory (mem0) once we choose how to run Python-backed servers org-wide.

## Source of truth (links)
- ADR-0071 (deferred-batch note), `docs/opencode-well-known.md`
- https://github.com/mem0ai/mem0 · https://github.com/SHSharkar/MCP-Mathematics · https://github.com/NameetP/pdfmux

## Current Behavior
Not present. ADR-0071 explicitly excluded them pending a packaging decision.

## Expected Behavior
Each Python-backed server runs via an agreed mechanism (per-user `uvx` local, or an in-cluster sidecar/gateway route) and is gated behind a dedicated subagent (e.g. `@pdf`, `@math`).

## Acceptance Criteria
- [ ] Decision recorded (ADR): `uvx` local vs in-cluster sidecar/gateway, per server.
- [ ] pdfmux's Python dep + LLM-key requirement resolved (which key, whose).
- [ ] Servers added behind subagent(s) with deny-baseline + allow.
- [ ] Renders + lints; capability verified.

## Out of Scope
- The npm-only and token-gated batches (separate tickets).

## Technical Context
`uvx` is not bare-npx; pdfmux needs `pip install pdfmux` + GEMINI/ANTHROPIC/OPENAI key. mem0 typically needs a store/server.

## Risks
- New Python packaging surface; pdfmux key provisioning; mem0 statefulness.

## Test Plan
- Validate the chosen run mechanism; exercise a PDF extraction + a math op end-to-end.

## Human accountable owner
@stephane-segning

## AI Usage Declaration
AI drafted this ticket from the ADR-0071 deferral; human owns intent + verification.