feat(shell): multi-shell per workspace + fix WS disconnect killing bash

BREAKING: every shell operation now requires a shellId. The Manager
previously keyed by workspaceID alone (one bash per workspace). It now
keys by (workspaceID, shellID) where shellID is a UUID returned by
Start.

Fixes the long-standing bug where the WS handler's closeAll called
stdin.Close() and killed bash when a WS client disconnected. The
Session owns the pty file; the WS handler no longer closes it. The
pty is only closed by Manager.Stop (explicit) or by captureOutput
when the process naturally exits (EOF).

- internal/shell/manager.go: Manager interface gains List and every
  method takes shellID; storage becomes
  map[workspaceID]map[shellID]*Session; Start returns (shellID, err)
  via uuid.NewString; Resize/Status/ExitStatus/Subscribe/Stdin/Stop
  route by shellID; new List(workspaceID) returns ShellInfo[] in
  creation order.
- internal/shell/session.go: Session gains ShellID + CreatedAt; Status
  type gains ShellID.
- internal/shell/manager_test.go: updated existing tests for new
  signatures; added TestShellMultiInstance (two shells in one
  workspace, no output cross-talk, independent stop, List behavior).
- internal/service/shell_service.go: wrappers carry shellID; new
  List method.
- internal/service/workspace_service.go: auto-start captures/logs
  shellID; Delete iterates and stops all workspace shells.
- internal/api/shell_handler.go: WS closeAll drops stdin.Close();
  start/restart return 201 with {shellId, pid, ...}; new list handler;
  stop/resize take shellId in body.
- internal/api/router.go: GET /api/workspaces/:id/shell (list).
- internal/model/shell.go: new ShellStartResponse, ShellInfo,
  ShellListResponse, ShellStopRequest, ShellRestartRequest; updated
  ShellStatusResponse + ShellResizeRequest to carry shellId.
- go.mod/go.sum: github.com/google/uuid.

E2E:
- workspace create -> 1 auto shell
- start 2 more -> 3 shells in list
- stop 1 -> 2 shells in list
- WS connect -> send cmd -> disconnect -> WS reconnect -> send cmd ->
  response OK, no [process exited] banner

internal/shell/session.go

@@ -4,11 +4,13 @@ import (
 	"io"
 	"os/exec"
 	"sync"
+	"time"
 )
 
 // Status represents the status of a shell for a workspace.
 type Status struct {
 	WorkspaceID string `json:"workspaceId"`
+	ShellID     string `json:"shellId,omitempty"`
 	Running     bool   `json:"running"`
 	PID         int    `json:"pid,omitempty"`
 }
@@ -21,12 +23,14 @@ type ExitInfo struct {
 
 // Session holds the runtime state of a started shell.
 type Session struct {
+	ShellID     string
 	WorkspaceID string
 	Root        string
 	Cmd         *exec.Cmd
 	Stdin       io.WriteCloser
 	Subscribers map[Subscription]struct{}
 	Exit        ExitInfo
+	CreatedAt   time.Time
 	mu          sync.Mutex
 }