docs: extract installation guide from README into docs/installation/

[Rayhan Hossain (hossain-rayhan)]

Summary

The README.md had grown to ~350 lines, with ~300 of those dedicated to install/uninstall/troubleshooting content. This PR extracts that content into a dedicated docs/installation/ tree so the root README focuses on what the kit is and points to the install docs for the how.

No content was rewritten — sections were moved verbatim and the per-OS guides were lifted as-is from the README.

What changed

README.md — Installation section reduced from ~300 lines to a short summary + per-OS table + a single link to the docs index:

## Installation
<one-paragraph summary>

| OS | Guide |
|---|---|
| macOS | docs/installation/macos.md |
| Linux | docs/installation/linux.md |
| Windows | docs/installation/windows.md |

For uninstall, troubleshooting, manual install, ... see docs/installation/.

New docs/installation/ tree:

File	Source in old README
README.md	New index page (summary + per-OS table + reference table)
macos.md	"Step-by-step: macOS"
linux.md	"Step-by-step: Linux"
windows.md	"Step-by-step: Windows"
what-gets-installed.md	"What gets installed"
requirements-and-flags.md	"Requirements summary" + "Common flags"
verify.md	"Verify it worked"
uninstall.md	"Uninstall"
troubleshooting.md	"Troubleshooting"
manual-install.md	"Manual install (no script)"
skills-only.md	"Universal one-liner — skills only"
plugin-marketplaces.md	"Coming soon: per-agent plugin / extension marketplaces"
updating.md	"Updating the kit"

What did not change

• Installer scripts (install.sh, install.ps1) — kept at repo root because the published curl one-liners hardcode raw.githubusercontent.com/.../main/install.sh.
• README sections kept: intro, Public Preview banner, Repo Structure, Configuration, Compatibility.
• No new content added. All section copy is verbatim from the previous README. New install docs (e.g. container-based install) will land in a follow-up PR after testing.

Verification

• All internal anchor links updated (#verify-it-worked → docs/installation/verify.md, etc.).
• Per-OS guides link to verify.md in the same folder.
• Skills-only / plugin-marketplaces / updating / manual-install docs use relative ../../README.md and ../../skills/... paths so they still resolve from the new location.
• grep -r '#verify-it-worked\|#troubleshooting\|#uninstall' . returns no broken references.