chore(scripts): adopt the unified install-script template

[trakhimenok]

The ingitdb-ws marketing site now ships unified install scripts at
ingitdb.com/install/get-cli and
ingitdb.com/install/get-cli.ps1,
copied from the canonical template defined at
sneat-co/ai-marketplace/CLIs.md.

Per that convention, the scripts should live in this repo at
scripts/install.sh and scripts/install.ps1. The marketing site
would then fetch them at build time, matching the
specscore-cli pattern.

Until that lands, ingitdb-ws hosts the scripts locally as
public/install/get-cli{,.ps1}. The scripts also encode an OS-aware
fallback for the three checksum manifests produced by this repo's
release pipeline:

Job	Checksum file
Linux	checksums.txt
macOS	checksums-darwin.txt
Windows	checksums-windows.txt

The unified template expects a single ${BIN_NAME}_${VER_NO_V}_checksums.txt.
The fallback is a documented hack — converging on a single manifest is
the long-term fix.

Acceptance criteria for closing this issue:

• scripts/install.sh lives in this repo, adapted from the
  datatug template.
• scripts/install.ps1 lives in this repo, adapted from the
  datatug template.
• Release pipeline produces a single unified
  ${BIN_NAME}_${VER_NO_V}_checksums.txt (preferred), OR the install
  scripts keep the three-manifest fallback (acceptable interim).
• ingitdb-ws build pipeline switches to fetch the scripts from
  this repo at build time.
• PR against sneat-co/ai-marketplace/CLIs.md flips ingitdb's
  curl/PowerShell cells from 🚧 to ✅.