feat(workspace): add repos JSONB field for GitHub repository URLs

Add a `repos` JSONB column to the workspace table for storing
associated repository URLs and descriptions. This enables the daemon
to clone repos and set up git worktrees for agent task execution.

Structure: [{"url": "https://github.com/org/repo", "description": "..."}]

- Migration 014: adds repos column with default '[]'
- UpdateWorkspace query: supports updating repos
- Workspace API: returns repos in GET, accepts in PUT

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/pkg/db/queries/workspace.sql

@@ -23,6 +23,7 @@ UPDATE workspace SET
     description = COALESCE(sqlc.narg('description'), description),
     context = COALESCE(sqlc.narg('context'), context),
     settings = COALESCE(sqlc.narg('settings'), settings),
+    repos = COALESCE(sqlc.narg('repos'), repos),
     updated_at = now()
 WHERE id = $1
 RETURNING *;