docs(install): rewrite install docs as a literal, audience-scoped step-by-step

[costezki]

Summary

• Single install hierarchy: README.md → docs/environment-setup.md (CLI-agnostic canon) → setup-claude.md/setup-opencode.md (literal numbered steps, one command/link each, ending in a real verify step).
• Separated audiences that had drifted together: mapping.md, compatibility.md, body-agnosticism-audit.md are labeled contributor-facing (not install docs); dual-cli/README.md reframed as that contributor annex instead of a competing install entry point.
• New docs/dual-cli/mcp-setup.md: step-by-step for all 7 referenced MCP servers (GitNexus, Atlassian, Google Workspace, Odoo, Neo4j, MongoDB, context7) with real package-install commands, not just config templates. Linked from environment-setup.md.
• Fixed factual gaps found while writing this: stream-coding had no install command anywhere (now: real upstream + clone/copy commands); superpowers/ponytail were documented as a .claude/skills/ copy on opencode when both now ship native opencode plugins (fixed in compatibility.md and setup-opencode.md); setup-opencode.md was missing the mandatory-external-dependencies step entirely (added, matching the Claude side).
• setup-opencode.md's verify step now uses the real skillery-opencode status command (and mentions --dry-run) instead of a vague "ask the agent to list its skills" — cross-checked against Grzegorz's skillery-opencode CLI implementation to make sure every command/flag referenced actually exists.
• Archives the driving OpenSpec change (step-by-step-install-guide) and syncs the dual-cli-distribution spec's "Per-CLI documentation split" requirement to match what shipped.

Test plan

• Manual top-to-bottom read of both runbooks — no forward references, one action per step, ends in a verification step.
• Scripted check that every relative markdown link/anchor across the touched docs resolves.
• openspec validate --changes step-by-step-install-guide --strict and openspec validate --all --strict both pass.
• make validate (needs a local .venv, not available in this environment — please run in CI).

🤖 Generated with Claude Code