migrate(dev-tree): replace dev-lead symlink with !external resolver block

Phase 3a-PR-D of internal#77 (task #235). Completes the cross-repo
composition migration started by PR #5 (sibling-clone+symlink) and
the platform-side !external resolver shipped in molecule-core#105.

CHANGES
  - org.yaml: replace
        - !include dev-lead/workspace.yaml
    with
        - !external
            repo: molecule-ai/molecule-dev-department
            ref: main
            path: dev-lead/workspace.yaml
    Composition is now platform-side: at POST /org/import time the
    workspace-server fetches molecule-ai/molecule-dev-department at
    ref=main into <orgBaseDir>/.external-cache/, grafts the dev-lead
    subtree, and rewrites every files_dir to be cache-prefixed.

  - dev-lead symlink deleted. The post-suspension sibling-clone deploy
    contract (operator must keep both repos as siblings under
    /org-templates/) is no longer required. The platform fetches the
    subtree on demand.

  - .gitignore: add .external-cache/ entry. Operators see the cache
    populate during imports; we don't track it.

DEPLOYMENT IMPACT
  Operators running molecule-core PR #105 or later can import this
  template without operator-side molecule-dev-department clone
  (task #230 becomes obsolete). Operators running an older platform
  binary will see import fail with 'unknown !external tag' — visible
  error, not silent breakage.

  To pin a specific tag/SHA for production stability, change ref: main
  to e.g. ref: v1.0.0 (currently no tags published; landing this
  unlocks the option).

VERSIONING
  No DB schema change. No public API change. Pure org-template
  composition shape change. Existing imports of older snapshots of
  this template still work — !include + symlink were never the only
  composition mechanism, and operators who have the symlink can
  re-add it locally if they need to roll back.

VERIFIED LOCALLY 2026-05-08
  --- PASS: TestPRD_MigratedParentTemplateImports (0.55s)
  post-migration parent resolves to 39 workspaces
  (5 PM-tree + 6 Marketing-tree + 28 dev-tree, fetched from
  molecule-ai/molecule-dev-department@main via the production
  gitFetcher into .external-cache/).

FOLLOW-UP TASKS
  - Update molecule-dev-department's local-e2e-setup.sh to drop the
    symlink check (now that parent template doesn't ship one).
  - Decide fate of TestLocalE2E_DevDepartmentExtraction +
    TestLocalE2E_FilesDirConsumption (they tested the symlink-based
    composition; now skip gracefully because no template uses that
    shape — keep as opt-in regression coverage).
  - Tag molecule-ai/molecule-dev-department v1.0.0 + pin ref here.

Refs:
  internal#77 — extraction RFC (Phase 3a phasing in comment 1995)
  molecule-core#105 — !external resolver
  molecule-core#106 — !external integration + e2e tests
  task #235 (PR-D)
  task #230 — obsolete after this PR
  Hongming GO 2026-05-08 ('do PR-B/C/D')

.gitignore

@@ -19,3 +19,6 @@
 # Workspace auth tokens
 .auth-token
 .auth_token
+
+# Platform .external-cache/ (cross-repo subtree fetch cache, see internal#77)
+.external-cache/

dev-lead

@@ -1 +0,0 @@
-../molecule-dev-department/dev-lead

org.yaml

@@ -147,10 +147,15 @@ defaults:
 workspaces:
   - !include teams/pm.yaml
   - !include teams/marketing.yaml
-  # Dev tree extracted to molecule-ai/molecule-dev-department (internal#77).
+  # Dev tree composed via gitops `!external` resolver (internal#77 / molecule-core#105).
-  # The `dev-lead` symlink at this template's root resolves through the
+  # Platform fetches molecule-ai/molecule-dev-department at import time into a
-  # operator-side sibling-clone convention to molecule-dev-department/dev-lead/.
+  # content-addressable cache under <orgBaseDir>/.external-cache/. No operator-
-  # See README §Dev tree composition for the deploy invariant.
+  # side filesystem symlink, no sibling-clone deploy contract — the import is
-  - !include dev-lead/workspace.yaml
+  # self-contained. To pin a specific tag/SHA in production, change `ref: main`
+  # below (e.g. `ref: v1.0.0`).
+  - !external
+    repo: molecule-ai/molecule-dev-department
+    ref: main
+    path: dev-lead/workspace.yaml
 
 template_schema_version: 1