Add abstract service interface packages

@deepseek-ai/dsh-llm: provider-neutral content-block vocabulary
(merge-extensible maps), raw StreamChunk protocol, ToolSchema,
abstract LlmAdapter, LlmService adapter registry, BlockAssembler.

@deepseek-ai/dsh-session: event-sourced Session (append-only log,
deriveMessages; context/steering render as tagged envelopes),
SessionStore, session/event + awaited session/flush durability seam.

@deepseek-ai/dsh-system-prompt: ordered sections + tool-schema
providers; assemble() through the system-prompt/assemble waterfall.
Tool schemas are part of the assembly by design.

@deepseek-ai/dsh-tools: tool registry feeding schemas into the
assembly; execute() through the tools/execute waterfall (the single
sandbox/permission/hook seam).

@deepseek-ai/dsh-agent: Agent interface (send/steer/inject/abort,
spawn/fork TODO seams), AgentRegistry, and the full agent/* event
taxonomy so plugins never depend on the concrete loop.

packages/system-prompt/tests/system-prompt.spec.ts

@@ -0,0 +1,71 @@
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import SystemPrompt, { PromptAssembly, renderPrompt } from '@deepseek-ai/dsh-system-prompt'
+
+describe('SystemPrompt', () => {
+  it('assembles sections in order with dynamic text and collected tools', async () => {
+    const ctx = new Context()
+    await ctx.plugin(SystemPrompt)
+
+    ctx.systemPrompt.section({ name: 'persona', order: 0, text: 'You are DeepSeek Code.' })
+    ctx.systemPrompt.section({ name: 'cwd', order: 20, text: () => `cwd: /tmp` })
+    ctx.systemPrompt.section({ name: 'rules', order: 10, text: 'Be precise.' })
+    ctx.systemPrompt.tools(() => [{ name: 'echo', description: 'echo back', parameters: {} }])
+
+    const assembly = await ctx.systemPrompt.assemble()
+    expect(assembly.sections.map(s => s.name)).toEqual(['persona', 'rules', 'cwd'])
+    expect(assembly.tools).toEqual([{ name: 'echo', description: 'echo back', parameters: {} }])
+    expect(renderPrompt(assembly)).toBe('You are DeepSeek Code.\n\nBe precise.\n\ncwd: /tmp')
+  })
+
+  it('removes contributions when the contributing fiber is disposed (HMR safety)', async () => {
+    const ctx = new Context()
+    await ctx.plugin(SystemPrompt)
+
+    const fiber = await ctx.plugin(Object.assign((inner: Context) => {
+      inner.systemPrompt.section({ name: 'scoped', order: 0, text: 'scoped section' })
+      inner.systemPrompt.tools(() => [{ name: 'scoped-tool', description: '', parameters: {} }])
+    }, { inject: ['systemPrompt'] }))
+
+    expect((await ctx.systemPrompt.assemble()).sections).toHaveLength(1)
+    await fiber.dispose()
+    const assembly = await ctx.systemPrompt.assemble()
+    expect(assembly.sections).toHaveLength(0)
+    expect(assembly.tools).toHaveLength(0)
+  })
+
+  it('composes multiple system-prompt/assemble waterfall listeners in order', async () => {
+    const ctx = new Context()
+    await ctx.plugin(SystemPrompt)
+    ctx.systemPrompt.section({ name: 'base', order: 0, text: 'base' })
+
+    // Listener A appends a section, then delegates.
+    ctx.on('system-prompt/assemble', async (assembly: PromptAssembly, next) => {
+      assembly.sections.push({ name: 'from-a', order: 100, text: 'a' })
+      return next()
+    })
+    // Listener B (registered later, runs after A) sees A's contribution.
+    const seen: string[][] = []
+    ctx.on('system-prompt/assemble', async (assembly: PromptAssembly, next) => {
+      seen.push(assembly.sections.map(s => s.name))
+      return next()
+    })
+
+    const assembly = await ctx.systemPrompt.assemble()
+    expect(seen).toEqual([['base', 'from-a']])
+    expect(assembly.sections.map(s => s.name)).toEqual(['base', 'from-a'])
+  })
+
+  it('lets a waterfall listener short-circuit by not calling next()', async () => {
+    const ctx = new Context()
+    await ctx.plugin(SystemPrompt)
+    ctx.systemPrompt.section({ name: 'real', order: 0, text: 'real' })
+
+    ctx.on('system-prompt/assemble', async () => {
+      return { sections: [], tools: [] } satisfies PromptAssembly
+    })
+
+    const assembly = await ctx.systemPrompt.assemble()
+    expect(assembly.sections).toHaveLength(0)
+  })
+})