feat(shell): add parallel bash shell subsystem auto-started per workspace

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -44,6 +44,12 @@ workspace:
   root: "./workspaces"
 process:
   opencodeCommand: "opencode"
+
+shell:
+  command: "bash"
+  args:
+    - "-i"
+
 file:
   maxWriteBytes: 1048576
 gin:
@@ -60,6 +66,8 @@ log:
 | `CODESPACE_ADDR` | Listen address | `:8080` |
 | `CODESPACE_WORKSPACE_ROOT` | Workspace storage root | `./workspaces` |
 | `CODESPACE_OPENCODE_COMMAND` | OpenCode binary path | `opencode` |
+| `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` |
 | `CODESPACE_GIN_MODE` | Gin mode (`debug`, `release`, `test`) | `release` |
 | `CODESPACE_READ_TIMEOUT` | HTTP read timeout | `15s` |
@@ -107,6 +115,16 @@ All runtime logging uses slog; `fmt`, `log.Printf`, and the Gin default logger a
 | `GET` | `/api/workspaces/:id/process/status` | Get process running status and PID |
 | `GET` | `/api/workspaces/:id/process/ws` | WebSocket: stream process stdout/stderr, send stdin (text frames, raw bytes) |
 
+### Shell
+
+| Method | Path | Description |
+|---|---|---|
+| `POST` | `/api/workspaces/:id/shell/start` | Start interactive shell for workspace |
+| `POST` | `/api/workspaces/:id/shell/stop` | Stop interactive shell |
+| `POST` | `/api/workspaces/:id/shell/restart` | Restart interactive shell |
+| `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) |
+
 ### Health
 
 | Method | Path | Description |