feat(cli): extend run command with port, attach, session-id, on-complete, and json options

Implement all 5 CLI extension options for external orchestration:

- --port <port>: Start server on port, or attach if port occupied
- --attach <url>: Connect to existing opencode server
- --session-id <id>: Resume existing session instead of creating new
- --on-complete <command>: Execute shell command with env vars on completion
- --json: Output structured RunResult JSON to stdout

Refactor runner.ts into focused modules:
- agent-resolver.ts: Agent resolution logic
- server-connection.ts: Server connection management
- session-resolver.ts: Session create/resume with retry
- json-output.ts: Stdout redirect + JSON emission
- on-complete-hook.ts: Shell command execution with env vars

Fixes #1586

src/cli/run/session-resolver.ts

@@ -0,0 +1,64 @@
+import pc from "picocolors"
+import type { OpencodeClient } from "./types"
+import { serializeError } from "./events"
+
+const SESSION_CREATE_MAX_RETRIES = 3
+const SESSION_CREATE_RETRY_DELAY_MS = 1000
+
+export async function resolveSession(options: {
+  client: OpencodeClient
+  sessionId?: string
+}): Promise<string> {
+  const { client, sessionId } = options
+
+  if (sessionId) {
+    const res = await client.session.get({ path: { id: sessionId } })
+    if (res.error || !res.data) {
+      throw new Error(`Session not found: ${sessionId}`)
+    }
+    return sessionId
+  }
+
+  let lastError: unknown
+  for (let attempt = 1; attempt <= SESSION_CREATE_MAX_RETRIES; attempt++) {
+    const res = await client.session.create({
+      body: { title: "oh-my-opencode run" },
+    })
+
+    if (res.error) {
+      lastError = res.error
+      console.error(
+        pc.yellow(`Session create attempt ${attempt}/${SESSION_CREATE_MAX_RETRIES} failed:`)
+      )
+      console.error(pc.dim(`  Error: ${serializeError(res.error)}`))
+
+      if (attempt < SESSION_CREATE_MAX_RETRIES) {
+        const delay = SESSION_CREATE_RETRY_DELAY_MS * attempt
+        console.log(pc.dim(`  Retrying in ${delay}ms...`))
+        await new Promise((resolve) => setTimeout(resolve, delay))
+      }
+      continue
+    }
+
+    if (res.data?.id) {
+      return res.data.id
+    }
+
+    lastError = new Error(
+      `Unexpected response: ${JSON.stringify(res, null, 2)}`
+    )
+    console.error(
+      pc.yellow(
+        `Session create attempt ${attempt}/${SESSION_CREATE_MAX_RETRIES}: No session ID returned`
+      )
+    )
+
+    if (attempt < SESSION_CREATE_MAX_RETRIES) {
+      const delay = SESSION_CREATE_RETRY_DELAY_MS * attempt
+      console.log(pc.dim(`  Retrying in ${delay}ms...`))
+      await new Promise((resolve) => setTimeout(resolve, delay))
+    }
+  }
+
+  throw new Error("Failed to create session after all retries")
+}