ci: add auto-tag workflow for multi-module releases

[lIang70]

Summary

• Add .github/workflows/auto-tag.yml that automatically tags changed modules (plugin, sdk, sdkx) when PRs are merged to main. Tags are created in topological order (leaf modules first), and when sdk is tagged, sdkx/go.mod is automatically updated to the new version before tagging sdkx.
• Bump sdkx sdk dependency from v0.1.4 to v0.1.5 (corresponding to the fix in fix(sdk): coerce string board vars to numeric types before JSON config parsing #12).

Workflow behavior

merge to main (paths: sdk/**, sdkx/**, plugin/**)
  │
  ├─ Phase 1: tag leaf modules (plugin, sdk) if changed
  │
  ├─ Phase 2: if sdk was tagged, update sdkx/go.mod → commit → push
  │
  └─ Phase 3: tag sdkx if changed (own changes or cascaded go.mod bump)

• Only bumps patch version automatically; minor/major releases remain manual.
• concurrency: cancel-in-progress: false prevents tag races on rapid merges.
• Root module (go.mod) is intentionally excluded from auto-updates.

Test plan

• Verify workflow YAML syntax is valid
• Merge and confirm auto-tag runs correctly on next sdk/ or sdkx/ change

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