[codex] Add advisory install guide

[Conalh]

Summary

• add a copy-paste advisory install guide for external teams
• add an example taskbound-advisory.yml workflow pinned to the current released Action tag
• link the install guide from the README GitHub Action section
• add a regression test that keeps the install docs pinned to the package version and advisory defaults

Scope

Only touch README, docs/INSTALL.md, examples/taskbound-advisory.yml, and a test that verifies the install guide stays pinned to the current Action tag and advisory defaults.

Validation

• node --test test/adoption-docs.test.mjs
• npm test
• npm run build
• git diff --check
• local TaskBound review returned NONE for the PR scope