docs: add a crawlable shell compatibility guide

[Mresyzz]

Problem

GitHub Repository Search now finds the repository for the main category and failure queries, but the public Pages deployment had no crawlable standalone troubleshooting page. The root llms.txt was also not deployed because Pages publishes docs/.

What changed

• Added docs/command-not-found.html, a standalone page for shell script compatibility checker, cross-distro shell testing, Debian/Ubuntu/Alpine runtime testing, exit code 127, and command not found searches.
• Added a copyable GitHub Actions workflow, expected PASS/FAIL matrix, limitations, local CLI commands, and links to configuration/troubleshooting/security docs.
• Added deployed docs/llms.txt and docs/sitemap.xml.
• Linked the new guide from both Pages entrypoints.
• Restored the full category phrase in the repository description, Action description, and PyPI summary while keeping shell testing as supporting language.
• Added shell-script-compatibility as an explicit topic.

Verification

GitHub Repository Search API (default best match, first 20, no star/owner filters) currently reports:

Query	Result
runtime compatibility shell	OpsScript Gate #2
cross distro shell test	OpsScript Gate #1
Debian Ubuntu Alpine shell GitHub Action	OpsScript Gate #1
Debian Ubuntu Alpine runtime shell	OpsScript Gate #1
shell script compatibility checker	OpsScript Gate #5
command not found shell GitHub Action	OpsScript Gate #2
cross-distro shell testing	OpsScript Gate #1

HTML, XML, TOML, and metadata checks passed. Runtime behavior is unchanged.