Preserve nil-link items across pipeline rebuilds

[id774]

Summary

• FeedMaker.create_pipeline no longer drops items only because link is nil.
• FeedObject no longer fabricates placeholder title/link values.
• SubscriptionText now produces documented title-only / URL-only items.
• FilterBatch -> FilterLimit regression proves linkless composition.
• FilterImage implementation is unchanged; documentation now states responsibility correctly.

Reason

• The pipeline contract allows link == nil; a missing field is documented
  data, not an error state (see doc/REQUIREMENTS.md section 8).
• FilterBatch and FilterJoin produce legitimate linkless items by design.
• FeedMaker.create_pipeline's hidden next if feed.link.nil? silently
  dropped those items, breaking composability with downstream link-independent
  plugins such as FilterLimit.
• FeedObject's http://dummy / dummy placeholder defaults meant
  SubscriptionText's title-only and URL-only items disagreed with the
  contract documented in doc/PLUGINS.md's SubscriptionText table.

Tests

All commands below were run in this environment (Ruby 3.3.6) after
bundle install:

• bundle exec rspec spec/lib/automatic/feed_maker_spec.rb — 14 examples, 0 failures
• bundle exec rspec spec/plugins/subscription/text_spec.rb — 10 examples, 0 failures
• bundle exec rspec spec/plugins/filter/batch_spec.rb — 12 examples, 0 failures
• bundle exec rspec spec/plugins/filter/image_spec.rb — 3 examples, 0 failures
• bundle exec rake spec:lib — 110 examples, 0 failures
• bundle exec rake spec:plugins — 290 examples, 0 failures
• bundle exec rake spec — 409 examples, 0 failures
• gem build automatic.gemspec — succeeded (automatic-26.09.gem)
• bundle exec ruby -Ilib -e "require 'automatic'" — exit 0
• bundle exec bin/automatic --version — exit 0, prints 26.09
• bundle exec bin/automatic --help — exit 0

Required CI (Ruby 3.3 / 3.4 / 4.0) status will be confirmed once the checks
run on this pull request.

Compatibility

• Public call shapes of FeedMaker.generate_feed(hash) and
  FeedMaker.create_pipeline(items) are unchanged.
• Recipe format is unchanged.
• FilterImage's direct behavior (blanking link for non-image URLs) is
  unchanged; only its documentation wording was corrected.
• Explicit nil-link guards in link-required plugins (e.g. StoreFile,
  StorePermalink) are unchanged.
• Date fallback and item ordering (do_sort) behavior are unchanged.
• No dependency changes.
• VERSION remains 26.09; this is a correction that aligns the
  implementation with the already-documented pipeline contract.

Version History

Added one coherent entry to the existing unreleased v26.09 section of
doc/VERSIONS:

Preserve nil-link pipeline items and remove SubscriptionText placeholder
title and link values.

https://claude.ai/code/session_01CbtimCQAVdGhaEFXsEGKPK

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