Add the proofread skill and sweep repository prose

[Oleg-Melnik]

Summary

Adds a proofread skill that corrects English grammar, punctuation, and
spelling in project-owned comments and documentation, together with the style
catalog and file-ownership boundary it applies. Also lands the skill's first
repository-wide sweep and updates the agent policy for pull request titles.

The repository-wide sweep touches comment and Markdown prose only. Within that
sweep, no code, identifier, string literal, link target, or generated file
changed, and every affected source file stays balanced at equal insertions and
deletions.

Changes

Skill, command, and guidelines

• Add .agents/skills/proofread/ (SKILL.md, agents/openai.yaml) and the
  .claude/commands/proofread.md entry point. The skill runs in three modes:
  branch diff (default), all for a full sweep, or a path for a scoped sweep.
• Add .agents/guidelines/english-style.md, the catalog that defines what
  counts as an error and, for each topic, when to leave an occurrence alone.
• Add .agents/guidelines/project-owned-files.md, the boundary that keeps any
  sweep out of the config submodule and the files it distributes.
• Index the skill in .agents/skills/README.md.
• Update AGENTS.md to require pull request titles without a trailing period.

Prose sweep — 47 files

• Restrictive which to that where the clause identifies its antecedent,
  leaving non-restrictive and preposition-governed uses alone (~45 places).
• allows to <verb> to the gerund form required after allow without an
  object, for example allows to amend to allows amending (~20 places).
• Subject-verb agreement, missing articles, one hyphenation, several double
  spaces, and one dialect outlier (behaviour to behavior in a file that is
  otherwise American).
• Repair three editing artifacts: a duplicated word, a run-together token, and
  a clause left fused to its replacement.
• Refresh the copyright year to 2026 on every file touched, per AGENTS.md.

Version

• Bump chordsVersion to 2.0.0-SNAPSHOT.96 and regenerate the pom.xml and
  dependencies.md reports.