[Ticket]: Token-gated opt-in MCP batch — Notion, Figma, Tavily/Exa, Sentry, 21st.dev

Part of #549

## Type
Feature

## Summary
Add the **token-gated opt-in batch** from the persona research: **Notion** (cross-persona winner), **Figma**, **Tavily/Exa** (search), **Sentry**, **21st.dev Magic** — all npm/npx, free tiers, each needing a per-user env token.

## Intent
Offer high-value SaaS integrations as opt-in subagents; users who set the env var get them, others see graceful failure (scoped to subagents).

## Source of truth (links)
- ADR-0072 (deferred token-gated note), `docs/opencode-well-known.md`
- https://github.com/makenotion/notion-mcp-server · https://github.com/GLips/Figma-Context-MCP · https://github.com/exa-labs/exa-mcp-server · https://github.com/tavily-ai/tavily-mcp · https://github.com/getsentry/sentry-mcp · https://github.com/21st-dev/magic-mcp

## Current Behavior
Not present (deferred to keep keyless users friction-free).

## Expected Behavior
Each added as a `type: local` server reading its token via `{env:...}` passthrough, behind a dedicated subagent (e.g. `@notion`, `@figma`, `@search`, `@errors`, `@components`), denied on the primary.

## Acceptance Criteria
- [ ] Per server: exact npx command + env var verified against current npm.
- [ ] Subagent + deny-baseline + allow wired; primary delegation lines added.
- [ ] Doc note on which token each needs + how a user sets it.
- [ ] Renders + lints.

## Out of Scope
- Provisioning users' personal tokens (user-side).

## Technical Context
Tokens come from each user's own shell env (local servers), not cluster ESO. Notion is the highest-value (free integration token).

## Risks
- Noisy failures for keyless users (mitigated by subagent scoping).

## Test Plan
- With a test token set locally, verify each subagent's tools work; confirm keyless = graceful failure only on delegation.

## Human accountable owner
@stephane-segning

## AI Usage Declaration
AI drafted this ticket from the ADR-0072 research; human owns intent + verification.

Start by reading ADR-0072 and docs/opencode-well-known.md, then verify the current npm/npx commands and environment variables for each linked MCP server. Review the existing local-server and subagent configuration entry points before wiring the five integrations. Done means the commands and tokens are verified, subagent permissions and delegation are configured, documentation is added, and renders and lints pass.

索引モデルが issue の本文から書いたものです。