Extend Copilot instructions and reusable skills

[frjcomp]

Summary

Add a structured Copilot customization surface for repository work by expanding the core instructions and introducing reusable agent/skill documentation.

Changes

• extend .github/copilot-instructions.md with a maintainer quick index, command implementation policy, command coverage guidance, debugging playbooks, and maintenance rules
• add .instructions.md as a short routing layer for Copilot task selection
• add .github/AGENTS.md with documented modes for command implementation, new commands, docs sync, PR review fixes, and CI failure debugging
• add reusable skill docs for command implementation, command coverage, docs sync, new command creation, PR review fixes, and PR actions failure debugging
• clarify that config.NewCommandSetup and WithFlagBindings are a paired implementation pattern
• require new commands to include unit tests, e2e tests, and behavior-asserting e2e coverage for each command-defined flag

Notes

• documentation-only change
• no runtime code paths changed

Validation

• verified working tree was clean before PR creation
• checked changed-file scope with git diff --stat origin/main...HEAD
• ran diagnostics on the touched documentation files

[Copilot]

Pull request overview

This PR expands the repository’s Copilot customization/documentation surface by adding a routing entrypoint, defining “agent modes”, and introducing reusable “skill” playbooks, while extending the canonical .github/copilot-instructions.md with maintainer-focused guidance.

Changes:

• Added .instructions.md as a lightweight routing layer to the authoritative instructions and reusable skill docs.
• Added .github/AGENTS.md plus multiple .github/SKILLS/*.md documents to standardize common maintainer workflows (command work, docs sync, PR review fixes, CI failure debugging).
• Extended .github/copilot-instructions.md with a maintainer quick index and additional workflow/debugging/maintenance guidance.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
.instructions.md	Adds a maintainer quick-start router pointing to the canonical instructions, agent modes, and skill docs.
.github/copilot-instructions.md	Extends canonical Copilot instructions with an index and additional maintainer workflow/debugging guidance.
.github/AGENTS.md	Documents “agent modes” for common workstreams (command changes, docs sync, PR fixes, CI debugging).
.github/SKILLS/command-implementation.md	Adds a reusable checklist for implementing/modifying commands with the repo’s config binding conventions.
.github/SKILLS/command-coverage.md	Adds a reusable procedure for recursive platform-wide command updates to avoid partial rollouts.
.github/SKILLS/docs-sync.md	Adds a playbook for keeping docs/config references aligned with flag/key changes.
.github/SKILLS/new-command.md	Adds creation-specific guidance for new commands, including unit + e2e coverage expectations.
.github/SKILLS/pr-review-fixes.md	Adds a structured process for triaging review comments and applying targeted fixes with validation.
.github/SKILLS/pr-actions-failures-debug.md	Adds a structured process for reproducing and fixing CI failures locally with documentation of evidence.