ci: migrate workflows to shared reusable workflows

[JunyangHe]

Description

Replace repository-specific GitHub Actions implementations with callers for reusable workflows
in aws/agentcore-devx-devtools.

• Migrate Slack issue and open-PR notifications.
• Migrate PR security review, CI failure reporting, Strands command, restricted agent, and
  public sync workflows.
• Combine PR size and title checks into pr-size-title.
• Preserve existing triggers and workflow inputs.
• Use CodeBuild runners where required.
• Forward secrets using secrets: inherit.
• Pin reusables to commit SHA 458c0a684af0f9e3a013ec05cd23851def4f9cab.
• Remove temporary feature-branch test triggers.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other: CI workflow migration to shared reusable workflows

Testing

Validated all eight callers against their reusable workflow contracts, including YAML syntax,
required inputs, permissions, CodeBuild runner selection, secrets: inherit, triggers, and
commit-SHA references.

No application source code changed, so application tests were not run.

• I ran npm run test:unit and npm run test:integ
• I ran npm run typecheck
• I ran npm run lint
• If I modified src/assets/, I ran npm run test:update-snapshots and committed the
  updated snapshots (not applicable)

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly (no documentation changes required)
• I have added an appropriate example to the documentation to outline the feature, or no
  new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute
this contribution, under the
terms of your choice.

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	40.34%	15063 / 37339
🔵	Statements	39.6%	16056 / 40541
🔵	Functions	34.48%	2573 / 7461
🔵	Branches	33.77%	10044 / 29736

Generated in workflow #4219 for commit 7aafe61 by the Vitest Coverage Report Action

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)