fix(acp): parse session/update content as ContentBlock, not string

The ACP spec defines session/update content as a ContentBlock object
({type, text}) but we decoded it as a string. This dropped every
agent_message_chunk and left prompt responses empty. E2E against the
real opencode acp binary surfaced the bug.

- internal/acp/messages.go: add ContentBlock struct; Update.Content is
  now ContentBlock.
- internal/acp/client.go: handleNotification extracts Text when
  Content.Type == "text"; other types are logged at debug and dropped.
- internal/acp/client_test.go: NDJSON framing test sends content as a
  ContentBlock object and reads .Text.
- README.md: document /acp/* routes + CODESPACE_OPENCODE_ARGS env.

Verified end-to-end: POST /acp/prompt returns text="Hi there!", 4
messages in history (1 user + 3 agent chunks), 0 invalid session/update
log entries.

Conversation: 019f357d-4236-7010-949c-e61067618d42

README.md

@@ -8,7 +8,8 @@ A Go backend skeleton for managing local workspaces and file operations with pro
 - File read/write/list/mkdir/remove/rename/stat through a `FileSystem` interface with path-escape protection.
 - Workspace root deletion protection — cannot remove the workspace root directory through the file API.
 - Configurable per-file write size limit — oversized writes are rejected early before hitting disk.
-- OpenCode process start/stop/restart/status per workspace.
+- OpenCode process start/stop/restart/status per workspace (launched with `opencode acp` by default; raw stdio is still piped through the process WebSocket for backward compatibility).
+- **ACP (Agent Client Protocol) client** — minimal JSON-RPC over NDJSON against the running opencode process. Exposes `initialize` / `session/new` / `session/prompt` / `session/cancel`. Serves `fs/read_text_file` and `fs/write_text_file` from the workspace filesystem. The web UI surfaces this as a chat panel alongside the terminal.
 - HTTP API using [Gin](https://github.com/gin-gonic/gin) for routing with JSON responses.
 - Explicit `http.Server` composition with configurable timeouts and max header bytes.
 - Structured logging using Go standard library `log/slog` with JSON/text format and level control.
@@ -44,6 +45,8 @@ workspace:
   root: "./workspaces"
 process:
   opencodeCommand: "opencode"
+  args:
+    - "acp"
 
 shell:
   command: "bash"
@@ -66,6 +69,7 @@ log:
 | `CODESPACE_ADDR` | Listen address | `:8080` |
 | `CODESPACE_WORKSPACE_ROOT` | Workspace storage root | `./workspaces` |
 | `CODESPACE_OPENCODE_COMMAND` | OpenCode binary path | `opencode` |
+| `CODESPACE_OPENCODE_ARGS` | Comma-separated extra args passed to OpenCode (sets `process.args`) | `acp` |
 | `CODESPACE_SHELL_COMMAND` | Shell binary path | `bash` |
 | `CODESPACE_SHELL_ARGS` | Comma-separated shell arguments | `-i` |
 | `CODESPACE_FILE_MAX_WRITE_BYTES` | Max bytes per file write | `1048576` |
@@ -125,6 +129,19 @@ All runtime logging uses slog; `fmt`, `log.Printf`, and the Gin default logger a
 | `GET` | `/api/workspaces/:id/shell/status` | Get shell running status and PID |
 | `GET` | `/api/workspaces/:id/shell/ws` | WebSocket: stream shell stdout/stderr, send stdin (text frames, raw bytes) |
 
+### ACP (Agent Client Protocol)
+
+Sits on top of the OpenCode process started above. The ACP service starts the process on the first prompt if it isn't already running, performs the `initialize` + `session/new` handshake, and accumulates the agent's responses until the `session/prompt` reply arrives.
+
+| Method | Path | Description |
+|---|---|---|
+| `GET` | `/api/workspaces/:id/acp/status` | Observational status: `{workspaceId, ready, sessionId, running, pid, error}`. Does **not** start the process. |
+| `GET` | `/api/workspaces/:id/acp/history` | Accumulated conversation: `{sessionId, messages: [{role, text, time}]}`, oldest first. In-memory only — cleared when the process restarts. |
+| `POST` | `/api/workspaces/:id/acp/prompt` | Body `{"content": "..."}`. Response `{"sessionId, stopReason, text}`. Starts the process + session on demand. Synchronous (waits up to 5 min). |
+| `POST` | `/api/workspaces/:id/acp/cancel` | Best-effort interrupt of an in-flight prompt. |
+
+`terminal/*` and other unsupported agent-initiated requests return JSON-RPC `Method not found` (`-32601`).
+
 ### Health
 
 | Method | Path | Description |
@@ -173,6 +190,14 @@ curl -s 'http://localhost:8080/api/workspaces/user1/files/read?path=main.go'
 curl -s 'http://localhost:8080/api/workspaces/user1/files/stat?path=main.go'
 ```
 
+### Send a prompt to the agent
+
+```sh
+curl -s -X POST http://localhost:8080/api/workspaces/user1/acp/prompt \
+  -H 'Content-Type: application/json' \
+  -d '{"content":"list the files in this workspace"}'
+```
+
 ## Test
 
 ```sh