Remove Git::Repository#lib in v6.0.0 #1769

Parent

Part of the v6.0.0 roadmap, #1717.

What to build

v6.0.0 no longer ships Git::Repository#lib. In v5.x it warns and returns self so that v4.x g.lib.some_method calls forward to the facade. After this change g.lib raises NoMethodError; callers use the facade method directly, as the "Git::Lib removed" entry in UPGRADING.md already maps for every affected call.

Per ADR-0007, the removal merges only after its deprecation and UPGRADING.md entry are in a normal release. Both shipped in v5.0.0; this ticket is additionally blocked on #1767 so every v5.x documentation fix ships before main becomes the v6.0.0 line.

Acceptance criteria

• Gate check recorded on this issue: v5.0.0 shipped the warning and the UPGRADING.md entry; Release v5.4.1 with every remaining v5.x deprecation documented in UPGRADING.md #1767 is closed
• Git::Repository#lib is removed along with its unit spec examples
• The stale lib/git/lib.rb entry under Metrics/ClassLength in .rubocop_todo.yml is removed (the file has not existed since v5.0.0)
• The "Upgrading to v6.0.0" section of UPGRADING.md states that #lib is gone, and the v5.x "Git::Lib removed" tables stay as the migration reference for callers upgrading from 4.x
• Commit marked breaking (! and a BREAKING CHANGE: footer)

Blocked by

• Release v5.4.1 with every remaining v5.x deprecation documented in UPGRADING.md #1767 (v5.4.1 shipped)