feat(skills): Add multi-repo skill for cross-repository orchestration

.claude/data/multi-repo/dependencies.yaml

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+# Multi-Repository Dependency Graph
+# This file tracks which repositories depend on each other
+#
+# Usage:
+#   - Add your repositories under 'repositories:'
+#   - List dependencies in 'depends_on:' for each repo
+#   - Document exposed interfaces in 'exposes:'
+#
+# The skill uses this graph to:
+#   - Detect breaking changes before they propagate
+#   - Determine correct merge order for linked PRs
+#   - Identify affected repos when interfaces change
+
+version: "1.0"
+last_updated: null
+
+# Repository dependency graph
+# Format:
+#   org/repo-name:
+#     depends_on:
+#       - repo: org/other-repo
+#         type: api|library|schema|data
+#         version: ">=1.0"  # semver constraint (optional)
+#     exposes:
+#       - type: api|library|schema|data
+#         name: Human-readable name
+#         contract: path/to/contract/file (optional)
+
+repositories:
+  # Example:
+  #
+  # my-org/api-server:
+  #   depends_on: []
+  #   exposes:
+  #     - type: api
+  #       name: REST API v2
+  #       contract: openapi.yaml
+  #
+  # my-org/web-client:
+  #   depends_on:
+  #     - repo: my-org/api-server
+  #       type: api
+  #       version: ">=2.0"
+  #   exposes: []

.claude/data/multi-repo/linked-prs.yaml

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+# Linked Pull Request Sets
+# Tracks PRs that should be merged together or in sequence
+#
+# Usage:
+#   - Each 'linked_set' groups related PRs across repos
+#   - 'merge_order' determines sequence (lower = earlier)
+#   - Status tracks progress: open -> approved -> merged
+
+version: "1.0"
+
+# Active linked PR sets
+# Format:
+#   - id: unique-identifier
+#     description: What this set accomplishes
+#     created: ISO timestamp
+#     status: pending|merging|completed|failed
+#     prs:
+#       - repo: org/repo-name
+#         pr: PR number
+#         branch: branch name
+#         status: open|approved|merged|closed
+#         merge_order: 1, 2, 3...
+
+linked_sets: []
+# Example:
+#
+# linked_sets:
+#   - id: "oauth2-implementation"
+#     description: "OAuth2 authentication across all services"
+#     created: "2025-11-25T10:00:00Z"
+#     status: "pending"
+#     prs:
+#       - repo: my-org/api-server
+#         pr: 123
+#         branch: feat/oauth2-backend
+#         status: "approved"
+#         merge_order: 1
+#       - repo: my-org/web-client
+#         pr: 456
+#         branch: feat/oauth2-frontend
+#         status: "open"
+#         merge_order: 2
+#       - repo: my-org/mobile-app
+#         pr: 789
+#         branch: feat/oauth2-mobile
+#         status: "open"
+#         merge_order: 2

.claude/skills/multi-repo/README.md

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+# Multi-Repository Orchestration Skill
+
+Coordinate atomic changes across multiple repositories when features span repo boundaries.
+
+## Quick Start
+
+1. **Initialize the dependency graph:**
+
+   ```
+   "Initialize multi-repo dependencies for this project"
+   ```
+
+2. **Add a dependency:**
+
+   ```
+   "Add dependency: my-org/web-client depends on my-org/api-server"
+   ```
+
+3. **Check for breaking changes:**
+
+   ```
+   "Check if my current changes break any dependent repositories"
+   ```
+
+4. **Create linked PRs:**
+   ```
+   "Create linked PRs for this feature across api-server and web-client"
+   ```
+
+## Directory Structure
+
+```
+.claude/
+  skills/multi-repo/
+    SKILL.md     # Skill definition
+    README.md    # This file
+  data/multi-repo/
+    dependencies.yaml   # Repository dependency graph
+    linked-prs.yaml     # Active linked PR sets
+```
+
+## Dependency Graph Example
+
+```yaml
+# .claude/data/multi-repo/dependencies.yaml
+version: "1.0"
+repositories:
+  my-org/api-server:
+    depends_on: []
+    exposes:
+      - type: api
+        name: REST API v2
+        contract: openapi.yaml
+
+  my-org/web-client:
+    depends_on:
+      - repo: my-org/api-server
+        type: api
+        version: ">=2.0"
+    exposes: []
+```
+
+## Common Use Cases
+
+### Detecting Breaking Changes
+
+Before modifying a public interface:
+
+```
+"I'm about to change the User schema in api-server.
+What repos will be affected?"
+```
+
+The skill will:
+
+1. Identify dependents from the graph
+2. Check if they use the changed interface
+3. Report impact and recommendations
+
+### Coordinated Feature Release
+
+For features spanning multiple repos:
+
+```
+"Implement feature X across api-server, web-client, and docs"
+```
+
+The skill will:
+
+1. Create feature branches in each repo
+2. Create PRs with cross-references
+3. Track them as a linked set
+4. Guide merge order based on dependencies
+
+### Merge Coordination
+
+When ready to merge a linked set:
+
+```
+"Merge the oauth2-implementation linked PRs"
+```
+
+The skill will:
+
+1. Verify all PRs are approved
+2. Merge in dependency order (upstream first)
+3. Wait for CI between merges
+4. Report completion status
+
+## Integration
+
+### With Worktree Manager
+
+Uses worktree-manager for local multi-repo operations:
+
+- Creates worktrees for each local repo
+- Manages parallel development
+- Cleans up after completion
+
+### With PM Architect
+
+For project management integration:
+
+- Linked PRs map to workstream items
+- Breaking changes feed into roadmap
+- Cross-repo work tracked in backlog
+
+## Limitations
+
+- Dependency graph is workspace-local (not centralized)
+- Requires `gh` CLI authentication for PR operations
+- Breaking change detection is heuristic-based
+- Circular dependencies not supported
+
+## Philosophy
+
+- **Ruthless Simplicity**: YAML files, no database
+- **Zero-BS**: All operations work today
+- **Modular**: Self-contained, optional PM integration