feat: update Copilot tools to use gpt-5-mini model by default

[KevinDeBenedetti]

Changes

• Changed default model in various components to gpt-5-mini for improved performance and cost-effectiveness.
• Updated GitHub Actions workflow to reflect the new model.
• Added new documentation for Copilot Chat & Session Management and PR Summary Workflow.
• Enhanced tests to ensure compatibility with the new default model.

[github-actions]

🤖 Copilot PR Summary

• Added AI-powered PR template auto-fill: new workflow posts suggested PR description fields on PR open and a PR template file to guide authors.
• Added/updated automated PR summary workflow and action: improved Node setup (Node 22), adjusted Bun run path, and switched defaults to a cost-friendly model (gpt-5-mini).
• New TemplateService and generate tooling: generate now supports a "template" type that analyzes diffs and returns structured template suggestions.
• Centralized DEFAULT_MODEL ("gpt-5-mini") and updated code/tests to use it.
• Large docs refresh: guides for chat/sessions, PR summary workflow, and template autofill added.

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Generated by Copilot Tools  ·  Model: gpt-5-mini

[github-actions]

🤖 Copilot PR Summary

Release notes

• Adds AI-powered PR template auto-fill: automated suggestions for PR descriptions when a PR is opened.
• Improves PR summary automation with a dedicated workflow that posts up-to-date summaries to PRs.
• Switches default model to a faster, cost‑effective model (gpt-5-mini) across the tools and updates related tests.
• Minor GitHub Actions fixes (Node setup and a bun invocation path) to make CI more reliable.
• Adds comprehensive docs for chat/sessions, PR summary workflow, and template auto-fill.

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Generated by Copilot Tools  ·  Model: gpt-5-mini