Audit cleanup: drop mustache, remove dead code, shrink helpers

[kplawver]

Summary

• Drops the mustache gem in favor of stdlib ERB for prompt templates — one less runtime dependency, same <%= %> substitution and conditional blocks.
• Removes dead code surfaced by a repo-wide over-engineering audit: the never-called IssueIdGenerator, the unused summary generation path (PromptBuilder#summary, the summary template, summary_requirements config var, and Report#summary + its renderer branches), the unread mention_triggers / collapse_previous_code_review_comments config flags, the one-line LlmClient#complete delegator, redundant require 'set' (autoloaded since Ruby 3.2), and a redundant AffectedRange#initialize override.
• Shrinks a few spots: inlines apply_string_overrides, collapses Reviewer#parse_response helpers, simplifies the CLI repo_owner/repo_name helpers, and updates the architecture docs (AGENTS.md / CLAUDE.md) to reference ERB.

Test plan

• bundle exec rake — RuboCop clean (46 files), RSpec 115 examples, 0 failures.
• Gemfile.lock no longer references mustache.

Notes

• The IO::Buffer is experimental warning during reviewer_spec is pre-existing (emitted by the async gem during the parallel review); it is not introduced by the ERB migration.

[rubyrt]

[Critical] ERB#result_with_hash is not a standard Ruby method — will raise NoMethodError at runtime

The code calls ERB.new(...).result_with_hash(...), but result_with_hash does not exist in Ruby's standard ERB library. The ERB API provides result(binding) and result_with_binding(binding), neither of which accept a plain Hash. This appears to be a direct copy-paste adaptation from the previous Mustache API (Mustache.render(template, hash)) without accounting for ERB's different rendering model. This will cause a NoMethodError at runtime whenever review or verify is called.

Tags: bug, compatibility

[kplawver]

If it wasn't, you wouldn't have run, friend.