ci: optimize pnpm install to prevent lock file conflicts

Changed CI pipeline to install dependencies only once in the install step.
All subsequent steps now reuse the installed node_modules instead of
reinstalling, which prevents ENOENT errors from concurrent pnpm lock file
operations.

- Only 'install' step runs 'pnpm install --frozen-lockfile'
- All other steps use 'corepack enable' and reuse existing dependencies
- Fixes ENOENT chown errors on lock.yaml temporary files

Refs #CI-woodpecker

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.woodpecker.yml

@@ -7,6 +7,8 @@ variables:
   - &install_deps |
     corepack enable
     pnpm install --frozen-lockfile
+  - &use_deps |
+    corepack enable
 
 steps:
   install:
@@ -17,7 +19,7 @@ steps:
   security-audit:
     image: *node_image
     commands:
-      - *install_deps
+      - *use_deps
       - pnpm audit --audit-level=high
     depends_on:
       - install
@@ -27,7 +29,7 @@ steps:
     environment:
       SKIP_ENV_VALIDATION: "true"
     commands:
-      - *install_deps
+      - *use_deps
       - pnpm lint || true # Non-blocking while fixing legacy code
     depends_on:
       - install
@@ -39,7 +41,7 @@ steps:
     environment:
       SKIP_ENV_VALIDATION: "true"
     commands:
-      - *install_deps
+      - *use_deps
       - pnpm --filter "@mosaic/api" prisma:generate
     depends_on:
       - install
@@ -49,7 +51,7 @@ steps:
     environment:
       SKIP_ENV_VALIDATION: "true"
     commands:
-      - *install_deps
+      - *use_deps
       - pnpm --filter "@mosaic/api" prisma:generate
       - pnpm typecheck
     depends_on:
@@ -60,7 +62,7 @@ steps:
     environment:
       SKIP_ENV_VALIDATION: "true"
     commands:
-      - *install_deps
+      - *use_deps
       - pnpm --filter "@mosaic/api" prisma:generate
       - pnpm test || true # Non-blocking while fixing legacy tests
     depends_on:
@@ -72,7 +74,7 @@ steps:
       SKIP_ENV_VALIDATION: "true"
       NODE_ENV: "production"
     commands:
-      - *install_deps
+      - *use_deps
       - pnpm --filter "@mosaic/api" prisma:generate
       - pnpm build
     depends_on: