docs: install tutorial PATH co-location + Pi GitHub URL sweep

[HenryLach]

Two coordinated documentation changes.

1. docs/tutorials/install.md — PATH setup co-located with install command

Option A (Global install) now shows the PATH setup snippets (bash/zsh AND PowerShell) directly under the pi install npm:taskplane command, rather than only in the 'taskplane not on your PATH' troubleshooting section several headings later.

A user reading top-to-bottom now hits the complete unified-update recipe at the natural decision point: install, then put pi's bin dir on PATH, and pi update is your single source of truth.

The 'taskplane not on PATH' section later in the file still has the three-option troubleshooting menu (npx, direct invocation, PATH addition) for users who reach that section without having read Option A.

2. Pi GitHub URL sweep — badlogic/pi-mono → earendil-works/pi

Pi's canonical repository is now at https://github.com/earendil-works/pi per Pi's own package.json:

"repository": {
  "type": "git",
  "url": "git+https://github.com/earendil-works/pi.git"
}

Four user-facing docs still pointed at the legacy badlogic/pi-mono location. Sweep update:

File	Refs updated
README.md	2
CONTRIBUTING.md	1
docs/specifications/cli/CLI-SPEC.md	1
docs/tutorials/install.md	1

All five replaced with the canonical https://github.com/earendil-works/pi URL. Verified post-change: zero badlogic/pi-mono references remain in the .md tree.

Out of scope (noticed during sweep)

There are still several @mariozechner/* package-name references in maintainer and specification docs:

• docs/maintainers/development-setup.md (loader mock docs)
• docs/maintainers/testing.md (loader mock docs)
• docs/specifications/cli/CLI-SPEC.md (peerDeps example)
• docs/specifications/framework/33-parallel-task-orchestrator.md (3x npm install -g @mariozechner/pi-coding-agent)
• docs/specifications/taskplane/code-quality-gates.md (already self-aware about staleness; flagged 'TP-191 will refresh')
• docs/specifications/taskplane/migrate-to-node-test-runner.md (test alias docs)

These are about the npm package scope (Pi was renamed from @mariozechner/pi-coding-agent to @earendil-works/pi-coding-agent at Pi v0.74.0), distinct from the GitHub URL change this PR addresses. Some are accurately documenting back-compat for legacy scopes (the test loader DOES need to recognize both for tests to run against historical Pi versions), others are stale. Worth a follow-up cleanup pass; not in scope for this PR per your request scope.

Validation

Gate	Result
npm run format:check	✅ pass
grep -rn 'badlogic/pi-mono' --include='*.md' .	✅ no matches
Install tutorial Option A reads coherently top-to-bottom	✅ visually verified

No code changes, no release needed.