feat(daemon): agent-driven repo checkout with bare clone cache

Agents now decide which repo to use based on issue context and check out
repos on demand via `multica repo checkout <url>`. Workspace repos are
cached locally as bare clones for fast worktree creation.

Key changes:
- Add repocache package for bare clone management (clone, fetch, worktree)
- Add `multica repo checkout` CLI command that talks to local daemon
- Add POST /repo/checkout endpoint on daemon health server
- Pass workspace repos metadata through register + task claim responses
- Remove pre-created worktrees from execenv (workdir starts empty)
- Update CLAUDE.md template to instruct agents to use `multica repo checkout`
- Pass MULTICA_DAEMON_PORT, WORKSPACE_ID, AGENT_NAME, TASK_ID env vars to agent

server/internal/handler/agent.go

@@ -81,6 +81,13 @@ func agentToResponse(a db.Agent) AgentResponse {
 	}
 }
 
+// RepoData holds repository information included in claim responses so the
+// daemon can set up worktrees for each workspace repo.
+type RepoData struct {
+	URL         string `json:"url"`
+	Description string `json:"description"`
+}
+
 type AgentTaskResponse struct {
 	ID             string         `json:"id"`
 	AgentID        string         `json:"agent_id"`
@@ -94,6 +101,7 @@ type AgentTaskResponse struct {
 	Result         any            `json:"result"`
 	Error          *string        `json:"error"`
 	Agent          *TaskAgentData `json:"agent,omitempty"`
+	Repos          []RepoData     `json:"repos,omitempty"`
 	CreatedAt      string         `json:"created_at"`
 	PriorSessionID string         `json:"prior_session_id,omitempty"` // session ID from a previous task on same issue
 }