refactor(examples): extract reusable logic into tested packages

Logic that lived under examples/ was outside the per-file 100% coverage
gate (examples/ are not workspaces) and, in the stdio-UI case, duplicated
across two examples. Move it into packages/ so it is gated and de-duped.

- packages/ui-stdio (new): unify the two diverged stdio-chat.ts copies into
  one @deepseek-ai/dsh-ui-stdio plugin (welcome/agent Config). A test-only
  I/O seam (createStdioChat(ctx, config, runtime)) keeps process streams out
  of the serializable config and makes every render/EOF/disposal branch
  unit-testable. Per-file 100%. echo/coding cordis.yml now load the package;
  both src/stdio-chat.ts deleted.
- packages/llm-replay (new): move examples/acp-agent/src/llm-replay.ts (+ its
  spec) here so its derive/parse/replay branches fall under the coverage gate.
  cordis.snapshot.yml + README rewired to the package name; added apply/env
  /assertNever/abort tests to reach per-file 100%.
- examples/{echo,coding}-agent: keyless Loader-path e2e smokes that boot the
  real cordis.yml (no key) — the guard a hand-mounted unit test cannot be for
  the unwrapExports/export-shape class (postmortem 0001). examples/AGENTS.md
  codifies the keyless+with-key smoke convention (keyless-by-nature exception
  for echo-agent).
- AGENTS.md: a scoped, removal-triggered pre-release stance (foundation over
  blast radius). packages/README.md: new rows + a FIXME to later regroup ALL
  packages into a hierarchy. Wiring: tsconfig paths/refs, publint, knip,
  module-graph.

Verified: typecheck, lint, test:coverage (887 tests, 100%), build, hygiene,
doc-sync, test:snapshot (10), test:e2e (6 keyless pass, with-key self-skip).

examples/echo-agent/src/stdio-chat.ts

@@ -1,60 +0,0 @@
-import { createInterface } from 'node:readline'
-import type { Context } from 'cordis'
-import type {} from '@deepseek-ai/dsh-agent'
-
-export const name = 'stdio-chat'
-export const inject = ['agents']
-
-/**
- * Minimal UI plugin: reads lines from stdin → agent.send(); renders the
- * agent's stream chunks and tool activity to stdout. Demonstrates that a UI
- * is "just a plugin" — it only consumes the agent/* event taxonomy.
- */
-export function apply(ctx: Context) {
-  ctx.on('agent/stream-chunk', (_agent, _turn, _step, chunk) => {
-    if (chunk.type === 'text-delta') process.stdout.write(chunk.text)
-  })
-
-  ctx.on('agent/turn-start', (agent, turn) => {
-    process.stdout.write(`\n[${agent.id} turn ${turn}] `)
-  })
-
-  ctx.on('agent/turn-end', () => {
-    process.stdout.write('\n> ')
-  })
-
-  ctx.on('session/event', (_session, event) => {
-    if (event.type === 'tool/call') {
-      const { name: toolName, arguments: args } = event.data
-      process.stdout.write(`\n  [tool call] ${toolName}(${args})`)
-    } else if (event.type === 'tool/result') {
-      const { content } = event.data
-      const text = content.filter(b => b.type === 'text').map(b => b.text).join('')
-      process.stdout.write(`\n  [tool result] ${text}\n  `)
-    }
-  })
-
-  ctx.effect(() => {
-    const reader = createInterface({ input: process.stdin })
-    reader.on('line', (line) => {
-      const text = line.trim()
-      if (!text) return
-      const agent = ctx.agents.get('main')
-      if (!agent) {
-        console.error('agent "main" is not running')
-        return
-      }
-      if (agent.status === 'running') {
-        agent.steer([{ type: 'text', text }])
-      } else {
-        agent.send([{ type: 'text', text }])
-      }
-    })
-    reader.on('close', () => {
-      // allow the process to exit when stdin ends (piped input)
-      setTimeout(() => process.exit(0), 200)
-    })
-    process.stdout.write('echo-agent ready. Type a message ("echo <text>" triggers the tool).\n> ')
-    return () => { reader.close() }
-  }, 'stdio-chat')
-}