Fix StoreFullText failure handling and documentation

[id774]

Summary

• StoreFullText now completes the database write before an item can continue downstream.
• Database write failures are no longer swallowed as warn-only continuation.
• The AI tutorial now distinguishes link-based stores from StoreDigest.
• Policy / test comments now point at the current HTTPS author URL and current invariant numbers.

Compatibility

• Recipe configuration is unchanged.
• Dependencies are unchanged.
• Version remains 26.09.
• Successful StoreFullText behavior and link/title deduplication are unchanged.
• Only the StoreFullText database-write failure path changes from warn-and-continue to terminating the run.

Version History

• v26.09 remains unreleased with Release Date: TBD.
• One new StoreFullText failure-handling bullet was added.
• Documentation/comment-only corrections were not split into release-history items.

Validation

• bundle exec rspec spec/plugins/store/full_text_spec.rb (with activerecord/sqlite3 installed via the optional store group): 6 examples, 0 failures, including the new "should raise when storing a new blog entry fails" example. The file ran in full, not skipped.
• bundle exec rake spec (full default suite, same environment): 498 examples, 0 failures.
• gem build automatic.gemspec: succeeded; the built .gem artifact was removed after the check.
• bundle exec ruby -Ilib -e "require 'automatic'": no exception.
• bundle exec bin/automatic --version and --help: both exit 0, output unchanged.
• Static consistency: no remaining http://id774.net, no remaining literal Invariant 6; git diff --check passes with no output.

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https://claude.ai/code/session_01V56jKPdCK34jVYsvaE2Fms

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