fix(host): producer-owned source kind for framing inject (dsh 0.1.7-rc.2)

[weibaohui]

Problem

On dsh 0.1.7-rc.2, every scheduled/manual task execution fails its opening turn:

[dsh-taskboard] turn error detail: {"message":"format v4 message requires a producer-owned source kind","code":"UNKNOWN"}

dsh 0.1.7-rc.2 retired the generic kind: 'plugin' message source. The v4 session format's source() validator now requires every durable message to carry a producer-owned source.kind — a non-empty string that is not 'plugin'. The opening inject() framing line used source: { kind: 'plugin', plugin: 'dsh-taskboard' }, so the v4 format rejects it and the whole turn fails.

Fix

src/host/execution.ts — replace the retired kind: 'plugin' with the plugin's own kind 'dsh-taskboard', and declare it in MessageSourceMap so the taskboard owns its attribution (matching the cordis-host-runner / runtime-context convention where each producer owns its source kind):

declare module '@deepseek-ai/dsh-llm' {
  interface MessageSourceMap {
    'dsh-taskboard': { kind: 'dsh-taskboard' }
  }
}
// ...
handle.agent.inject({
  ...
  source: { kind: 'dsh-taskboard' as const },  // was: { kind: 'plugin', plugin: 'dsh-taskboard' }
})

The followup() body already used kind: 'user' and is unchanged. tests/execution.spec.ts updated to assert the new kind.

Verification (target: dsh 0.1.7-rc.2)

• tsdown build (host + client) succeeds.
• vitest: 346/346 tests pass (including the execution specs that exercise the inject framing path).
• Built lib/host/execution.js carries kind: "dsh-taskboard" (no 'plugin').

The same class of bug + fix was confirmed in another scheduled-task plugin on 0.1.7-rc.2.