Zi v2.0.0

Zi v2.0.0 is the breaking milestone release from the reviewed next to
main promotion in #467.

Breaking change: message output

The message-output contract was rebuilt in
#457:

• safe automatic formatting is now the default
• normal messages end with a line feed unless -n is requested
• +zi-progress owns carriage-return progress rendering
• dynamic or raw text should use literal mode and --
• invalid options and descriptors return status 2
• contextual recognition is opt-in

Custom callers that depend on exact bytes, newlines, progress rendering, or
file-descriptor behavior should follow the updated
message API guide.

Security

• Stage archive extraction and reject traversal and unsafe symlink members
  (#441).
• Reject unsafe symlinks in GitHub directory snippets
  (#439).

Features and reliability

• Add transactional sparse Git support for GitHub /trunk/ directory
  snippets while retaining SVN for other hosts
  (#443).
• Make parallel updates deterministic
  (#453).
• Persist Plugin Standard callbacks by effective plugin ID
  (#454).
• Track and remove plugin-owned hook callbacks
  (#459).
• Preserve system MANPATH entries
  (#460).
• Stop the scheduler timer when no work remains
  (#466).

Validation

The promotion passed 135 pull-request checks and all post-merge main
workflows. Coverage included the full Zsh syntax and behavior matrix, exact
candidate clean startup, ZD compatibility and five integration suites, Trunk,
CodeQL, public-contract impact, and downstream consumer review.

Full comparison: v1.1.0...v2.0.0