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, PR automerge, Dependabot automerge, and public sync workflows.
• Add the shared PR size and title workflow.
• Preserve existing triggers and workflow inputs.
• Use GitHub-hosted Ubuntu runners.
• 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 seven callers against their reusable workflow contracts, including YAML syntax,
required inputs, permissions, runner selection, secrets: inherit, triggers, and commit-SHA
references.

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

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

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[agentcore-devx-automation]

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

[agentcore-devx-automation]

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

[agentcore-devx-automation]

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