docs(upgrade): clarify release compatibility and bootstrap path

[dnlrsls]

Linked issue

Closes #2010

PR type

• Documentation only

Summary

• State that upgrade is available on main from commit 96ff045, but not in published releases through v4.0.0.
• Explain why older launchers open upgrade as a prompt and provide a fresh-main-checkout bootstrap procedure.
• Keep Gentle Shell upgrade distinct from Pi package update, including the effect of returning to stable.

Changes

File	Change
README.md	Put release compatibility and the older-installation path before upgrade examples.
docs/readme-reference.md	Document availability, bootstrap commands, channel choices and unattributed-installation limitations.

Test plan

• git diff --check passed before committing.
• Structurally read back the complete documentation diff and link targets.
• Native assessment: passive, structural readback only, reviewDue false.
• Runtime tests and installer execution: not applicable; documentation only, no runtime changes.

Contributor checklist

• Linked approved issue bug(docs): documented upgrade command opens a prompt on Gentle Shell 1.1.0 #2010.
• Exactly one type label: type:docs.
• Shellcheck: not applicable; no scripts changed.
• Skill runtime tests: not applicable; no skills changed.
• Documentation updated.
• Conventional commit; no Co-Authored-By trailer.

Scope

Only README.md and docs/readme-reference.md; rollback removes these documentation additions without changing runtime behavior.

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📥 Commits

Reviewing files that changed from the base of the PR and between a6c539a and 9192898.

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📒 Files selected for processing (2)

• README.md
• docs/readme-reference.md

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• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

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