feat: workspace backend foundation (PR-1)

- Add workspaces table migration (2026_06_01_add_workspaces)
- Create Workspace model with repo_id, user_id, branch, path, status
- Add workspace_id nullable FK to ToolInstance
- Create GitService for clone/fetch/pull/branch_exists_remotely
- Create WorkspaceManager for create/delete/sync lifecycle
- Create workspace CRUD API with 409 handling for duplicates and instances
- Wire workspace routes into FastAPI app
- 17 tests passing (8 unit + 9 integration), 1 skipped

Quality gates: ruff clean

apps/api/src/models/tool_instance.py

@@ -14,6 +14,7 @@ if TYPE_CHECKING:
     from src.models.project import Project
     from src.models.tool_type import ToolType
     from src.models.user import User
+    from src.models.workspace import Workspace
 
 
 class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
@@ -60,8 +61,12 @@ class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
         UUID(), ForeignKey("config_profiles.id", ondelete="SET NULL"), nullable=True
     )
     ssh_key_ids: Mapped[list[str] | None] = mapped_column(JSON, nullable=True)
+    workspace_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(), ForeignKey("workspaces.id", ondelete="SET NULL"), nullable=True
+    )
 
     tool_type: Mapped["ToolType"] = relationship()
+    workspace: Mapped["Workspace | None"] = relationship()
     repository: Mapped["GitRepository"] = relationship()
     project: Mapped["Project"] = relationship()
     owner: Mapped["User"] = relationship()

apps/api/src/models/workspace.py

@@ -0,0 +1,50 @@
+"""Workspace model for persistent writable repo clones."""
+
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+from typing import TYPE_CHECKING
+
+from sqlalchemy import DateTime, ForeignKey, String, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from src.models.base import Base, TimestampMixin
+
+if TYPE_CHECKING:
+    from src.models.git_repository import GitRepository
+    from src.models.user import User
+
+
+class Workspace(Base, TimestampMixin):
+    """A persistent, writable local clone of a Git repository.
+
+    Users create workspaces explicitly, then start tool instances on them.
+    Multiple tool instances can share the same workspace.
+    """
+
+    __tablename__ = "workspaces"
+
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    repo_id: Mapped[uuid.UUID] = mapped_column(
+        ForeignKey("git_repositories.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    branch: Mapped[str] = mapped_column(String(255), nullable=False, default="main")
+    path: Mapped[str] = mapped_column(String(2048), nullable=False)
+    status: Mapped[str] = mapped_column(String(16), nullable=False, default="ready")
+    last_sync_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+
+    __table_args__ = (
+        UniqueConstraint("repo_id", "name", name="uq_workspace_repo_name"),
+    )
+
+    repository: Mapped[GitRepository] = relationship("GitRepository")
+    owner: Mapped[User] = relationship("User")