feat(plugin-api): expose minimal session/workflow orchestration

[theshwal]

Summary

Plugin API v2 extension. Plugins can now call a tiny host facade via
context.host (PluginHost) to:

• create sessions (context.host.sessions.create)
• stop sessions (context.host.sessions.stop)
• launch workflows (context.host.workflows.launch)

Design rationale

The facade reuses the session manager and the task-seeded workflow
launcher already used by the MCP / WS paths so we don't introduce a
second implementation. The isRunning cycle stays entirely managed
by OpenFox; we never expose SessionManager, launchWorkflowRun, or
setRunning.

Plugins get exactly three generic operations already used by the
native MCP / WS surfaces. The API is generic and reusable by any
plugin, not specific to a single consumer.

Public surface

interface PluginSessions {
  create(input: {
    projectId: string
    title?: string
  }): Promise<{
    sessionId: string
    workdir?: string
  }>
  stop(sessionId: string): void
}

interface PluginWorkflows {
  launch(input: {
    sessionId: string
    workflowId: string
    params?: Record<string, string>
    content?: string
    subGroup?: string
  }): void
}

interface PluginHost {
  readonly sessions: PluginSessions
  readonly workflows: PluginWorkflows
}

interface PluginContext {
  // ...
  readonly host?: PluginHost
}

Implementation

• src/plugin/index.ts: new interfaces + optional host? field on
  PluginContext (and on the tool context so RPC handlers see it too).
• src/server/plugins/host.ts: new currentHost field + setHost method;
  createContext snapshots host via a lazy getter so plugins loaded
  before setHost still see the new value.
• src/server/index.ts: setHost({sessions, workflows}) wired after the
  tasks launcher is initialised.

Tests

src/server/plugins/host.test.ts covers:

• context.host is absent when setHost was never called
• context.host is present when setHost was called before plugin load
• setHost propagates to existing contexts (lazy getter)
• setHost(undefined) removes the host from existing contexts

Verification

• npm run typecheck (server + web): 0 errors
• npm run lint: 0 errors (one targeted eslint-disable for the
  no-this-alias rule required by the lazy-getter pattern)
• npm run test:unit: 434 files / 5843 tests passing, 29 skipped, 0 failed

AI-Enhanced Development

AI Models: MiniMax M3

What is NOT in this PR

• SessionManager is not exposed
• launchWorkflowRun is not exposed
• setRunning is not exposed
• no host/port/token plumbing is leaked to plugins
• no second implementation of the business logic — the facade simply
  delegates to the existing primitives used by MCP / tasks