research: persist bound repo + shallow-clone on start_topic

This is the minimum viable version of the "agents actually work on
a repo" architecture. Full vision (isolated ZeroClaw container per
topic, dynamic agent provisioning inside, pause/resume, commit
gate) is real weeks of work — this closes the first, most-visible
gap so the ClawHDF5 topic can actually run against its codebase.

Backend

- Migration 0037: research_topics gets repo_id UUID (nullable, FK
  to repos ON DELETE SET NULL) and repo_workspace_path TEXT for
  the on-disk checkout location. Index on repo_id when set.

- research_topics::create takes repo_id: Option<Uuid>. get + list
  select it and repo_workspace_path. set_repo_workspace_path
  persists the path once the first clone lands.

- CreateTopicRequest accepts `repo: Option<TopicRepoRef>` — the
  same denormalized shape the wizard already sends. Only repo_id
  is authoritative; other fields are ignored (dead_code-allowed
  so serde still deserializes the full body).

- start_topic branches on topic.repo_id. When set, it calls
  ensure_repo_workspace:
    · resolves repo.clone_url + repo.default_branch
    · target path = CLAWMATES_RESEARCH_WORKSPACE_ROOT
                    // <topic_id> // repo (defaults under $TMPDIR)
    · runs `git clone --depth 1 --single-branch --branch <b>` via
      tokio::process. Reuses the checkout if .git already exists.
    · persists the path so re-starts skip the clone
    · runs `git ls-files` to sample the tree (first 60 entries,
      total count reported honestly so the prompt doesn't lie
      about coverage)
  All best-effort — a clone failure logs but still starts the run
  without repo context rather than aborting.

- build_coordinator_task takes Option<&RepoContext>. When present,
  the framing gets a REPO block (slug / path / branch / file
  sample) and a USING THE REPO section instructing the coordinator
  to ground every recommendation in a concrete file reference and
  never fabricate paths. The per-topology bodies are unchanged —
  the repo guidance sits above them so it applies to every shape.

What this unblocks / doesn't unblock

Unblocks: The coordinator prompt now knows the repo exists, where
it lives on disk, and what's in it. Even without file-editing
tools wired to the checkout, the coordinator can point spokes at
concrete modules and the final artifact can reference real files.
For a spec-shaped outcome like ClawHDF5's, that's the difference
between abstract advice and a spec grounded in the actual crates.

Does NOT unblock: The agents themselves editing files, running
tests, or committing. That requires either mounting the checkout
into the ZeroClaw sandbox or exposing a new MCP tool for
repo-scoped file ops — separate follow-up.

.sqlx/query-a40940823a2d1e3105140d59c513fd6c7b27430c4350bdce82ce4427c9954ee7.json

@@ -0,0 +1,95 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "SELECT id, workspace_id, title, description, outcome_kind, status,\n                created_by, created_at, updated_at, published_at, topology_kind,\n                repo_id, repo_workspace_path\n           FROM research_topics\n          WHERE id = $1 AND workspace_id = $2",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "id",
+        "type_info": "Uuid"
+      },
+      {
+        "ordinal": 1,
+        "name": "workspace_id",
+        "type_info": "Uuid"
+      },
+      {
+        "ordinal": 2,
+        "name": "title",
+        "type_info": "Text"
+      },
+      {
+        "ordinal": 3,
+        "name": "description",
+        "type_info": "Text"
+      },
+      {
+        "ordinal": 4,
+        "name": "outcome_kind",
+        "type_info": "Text"
+      },
+      {
+        "ordinal": 5,
+        "name": "status",
+        "type_info": "Text"
+      },
+      {
+        "ordinal": 6,
+        "name": "created_by",
+        "type_info": "Uuid"
+      },
+      {
+        "ordinal": 7,
+        "name": "created_at",
+        "type_info": "Timestamptz"
+      },
+      {
+        "ordinal": 8,
+        "name": "updated_at",
+        "type_info": "Timestamptz"
+      },
+      {
+        "ordinal": 9,
+        "name": "published_at",
+        "type_info": "Timestamptz"
+      },
+      {
+        "ordinal": 10,
+        "name": "topology_kind",
+        "type_info": "Text"
+      },
+      {
+        "ordinal": 11,
+        "name": "repo_id",
+        "type_info": "Uuid"
+      },
+      {
+        "ordinal": 12,
+        "name": "repo_workspace_path",
+        "type_info": "Text"
+      }
+    ],
+    "parameters": {
+      "Left": [
+        "Uuid",
+        "Uuid"
+      ]
+    },
+    "nullable": [
+      false,
+      false,
+      false,
+      false,
+      false,
+      false,
+      false,
+      false,
+      false,
+      true,
+      false,
+      true,
+      true
+    ]
+  },
+  "hash": "a40940823a2d1e3105140d59c513fd6c7b27430c4350bdce82ce4427c9954ee7"
+}