docs: make single sources of truth authoritative for remaining duplicated values

[id774]

Summary

The plugin catalogue's totals and per-status counts were already consolidated into doc/PLUGINS.md section 6 in a prior change, but several other current-derived values were still duplicated across README and the docs. This finishes that cleanup.

• Point runtime dependency counts (README, DEPLOYMENT.md) at automatic.gemspec instead of restating "four pure-Ruby gems".
• Point the continuously validated Ruby set (README, DEPLOYMENT.md, REQUIREMENTS.md, RELEASING.md, CI comments) at the matrix in .github/workflows/ci.yml instead of repeating "3.3, 3.4 and 4.0".
• Drop the framework's duplicated line-count claim in favor of BASIC_DESIGN.md/POLICY.md as the maintained responsibility boundary.
• Replace the bundled siteinfo snapshot's stale "newest entries are from 2013" narrative (doc/PLUGINS.md and full_feed.rb comments) with behavior that doesn't depend on the snapshot's age, matching the v26.09 refresh from a newer upstream snapshot.
• Replace hardcoded current AI model names in PLUGINS.md and AI_TUTORIAL.md examples with placeholders, since model availability is service-owned.
• Resolve the QUICKSTART.md "two optional gems" claim against the three gems its own install steps actually use, by deriving the count from the DEPLOYMENT.md dependency table instead of stating it twice.
• Update AI_TUTORIAL.md's "no chunking plugin" claim to reflect the new FilterLimit/FilterBatch plugins added in v26.09.
• Remove the Status column from DEPLOYMENT.md's optional-dependency table, leaving PLUGINS.md section 6 as the single source for support status.
• Replace remaining small-count prose ("two conventions", "three ways to set one up", step-number cross-references, etc.) with references to the actual section/table rather than a number that drifts.
• Shorten two doc/VERSIONS v26.09 bullets that exceeded this file's own line-length guideline, without changing their content.

Gemfile, automatic.gemspec, and the GitHub Actions workflow files receive comment-only changes; no executable Ruby, YAML, or dependency declaration changed. plugins/filter/full_feed.rb also receives comment-only changes. spec/doc/plugins_catalogue_spec.rb, VERSION, lib/, assets/, config/, spec/, test/, and Rakefile are untouched.

Test plan

• bundle exec rspec spec/doc/plugins_catalogue_spec.rb — 4 examples, 0 failures, no diff to this file
• bundle exec rake spec — 357 examples, 0 failures
• git diff --check — clean
• Confirmed only the 14 intended files changed, and the gemspec/Gemfile/workflow/full_feed.rb diffs are comment-only

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