chore: establish agent-first development ethos across project

[johntmyers]

Summary

Establishes the agent-first development identity across OpenShell — documentation, GitHub workflows, issue/PR templates, agent skills, and contributor guardrails. The project builds safe agent runtimes and is itself built by agents; this PR makes that visible and enforceable.

• Rewrites AGENTS.md, CONTRIBUTING.md, and README.md to lead with agent-first philosophy
• Adds issue templates (bug report, feature request) with mandatory agent diagnostic sections
• Adds PR template, CODEOWNERS, and a GitHub Actions workflow that gates issues missing agent investigation
• Creates two new skills (triage-issue, sync-agent-infra), removes the redundant github-rest-api skill, and updates three existing skills for template conformance

Related Issue

N/A — internal initiative to protect and codify the project's agent-first development model.

Changes

Documentation (Phase 1)

• AGENTS.md — Added project identity, architecture overview, workflow chains, issue/PR conventions, testing, and security sections. Removed redundant skills inventory (moved to CONTRIBUTING.md).
• CONTRIBUTING.md — Added agent-first philosophy, "Before You Open an Issue" gate, full skills table with workflow chains, and "When to / When NOT to Open an Issue" guidance.
• README.md — Added agent-first hero paragraph, alpha software callout, "Explore with your agent" subsection, and "Built With Agents" section.

GitHub Infrastructure (Phase 2)

• .github/ISSUE_TEMPLATE/bug_report.yml — Structured bug template requiring agent diagnostic section, environment details, reproduction steps, and checklist.
• .github/ISSUE_TEMPLATE/feature_request.yml — Design-first feature template requiring agent investigation, design proposal, and checklist.
• .github/ISSUE_TEMPLATE/config.yml — Disables blank issues, redirects questions to agents, redirects security to SECURITY.md.
• .github/PULL_REQUEST_TEMPLATE.md — Standard PR structure: Summary, Related Issue, Changes, Testing, Checklist.
• .github/CODEOWNERS — Broad ownership under @NVIDIA/openshell-codeowners.
• .github/workflows/issue-triage.yml — Deterministic gate that labels issues missing agent diagnostic sections with needs-agent-triage.

Agent Skills (Phase 3)

• Added triage-issue — Assesses community issues (single or batch mode), validates agent diagnostic quality, applies labels, and posts triage summaries.
• Added sync-agent-infra — Detects drift across AGENTS.md, CONTRIBUTING.md, skill files, workflow files, and agent definitions; generates fix plan.
• Removed github-rest-api — Redundant with the sandbox-deployed github skill in deploy/docker/sandbox/skills/github/.
• Updated create-github-issue — Aligned output format with new issue templates.
• Updated create-github-pr — Aligned PR description with new PR template.
• Updated build-from-issue — Added needs-agent-triage awareness, aligned PR output with template.

Agent Definitions (Phase 4)

• .opencode/agents/arch-doc-writer.md — OpenCode sub-agent for architecture documentation.
• .opencode/agents/principal-engineer-reviewer.md — OpenCode sub-agent for code review.
• .claude/README.md — Updated to document the .claude/skills → ../.agents/skills/ symlink.

Testing

• All changes are documentation, templates, YAML workflows, and skill markdown — no runtime code modified.
• Verified issue triage workflow YAML is valid GitHub Actions syntax.
• Pre-commit checks pass (no Rust/Python code changed).

Checklist

• Follows conventional commit format
• No secrets or credentials committed
• Documentation updated (AGENTS.md, CONTRIBUTING.md, README.md)
• Changes scoped to the issue at hand
• Pre-commit checks pass