Fix file URI and CLI input boundary handling

[id774]

Summary

• StoreFile now hands PublishAmazonS3 the actual saved path as an absolute, percent-encoded file: URI, instead of concatenating a possibly-relative path onto file:// (which could be misread as carrying an authority, and could not represent a path containing a space or other URI-reserved character).
• PublishAmazonS3 now decodes the URI path back to a local filesystem path before uploading, undoing the percent-encoding StoreFile applies.
• The feedparser subcommand now converts the URL validation failure Automatic::Http.uri already applies into the CLI's ordinary failure path (stderr diagnostic, exit 1), instead of letting it surface as an unprocessed exception with a backtrace.
• plugins/filter/join.rb's comment describing FeedMaker.create_pipeline's nil-link behavior no longer describes a code path this plugin doesn't call; it now just documents why this item's link is nil.
• The Gemfile comment for the Supported (external) plugin group no longer claims their specs exercise the external service directly rather than a double, which does not match how those specs are actually written.

Compatibility

• Recipe format, plugin interface, configuration keys/defaults, and pipeline shape are unchanged.
• No dependency was added, removed, or had its version constraint changed.
• Ruby support (>= 3.3.0) and the CI matrix (3.3 / 3.4 / 4.0) are unchanged.
• No unexpected internal exception is blanket-rescued: an internal ArgumentError raised from inside FeedParser.get_url (after URL validation passes) still propagates unconverted.

Version History

• The CLI correction is covered by the existing v26.09 "Harden the framework's execution boundary" bullet in doc/VERSIONS, which already describes fixing CLI contract drift; no new bullet was added for it.
• The StoreFile/PublishAmazonS3 handoff fix is recorded as one new v26.09 bullet: "Preserve local file paths when StoreFile hands file URIs to PublishAmazonS3."
• No release item was added for the comment-only corrections in plugins/filter/join.rb or Gemfile, or for the doc/PLUGINS.md clarifications.

Validation

• Targeted specs (spec/plugins/store/file_spec.rb, spec/plugins/publish/amazon_s3_spec.rb, spec/lib/automatic/cli_spec.rb): 45 examples, 0 failures.
• Full default suite (bundle exec rake spec): 423 examples, 0 failures (network spec not enabled).
• gem build automatic.gemspec: succeeded (version 26.09).
• require 'automatic': loads successfully.
• bin/automatic --version: exit 0.
• bin/automatic --help: exit 0.
• Manually verified feedparser against file:///etc/passwd, https:/feed, and http://[: each exits 1 with a one-line automatic: diagnostic on stderr and empty stdout.
• GitHub Actions Ruby 3.3 / 3.4 / 4.0: pending on this PR; will confirm once CI runs.

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https://claude.ai/code/session_017Zn5VetozA4NmGEUGLGik1

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