Ship only runtime files in the gem; add pack-contents spec

[minerva-sky]

Fixes #55.

What changed

• agentic.gemspec: the files clause is now an allowlist (lib/, exe/, sig/, LICENSE.txt, README.md, CHANGELOG.md) instead of a denylist over git ls-files, and bindir is set to exe (it defaulted to bin, so RubyGems synthesised a bin/agentic entry that does not exist).
• spec/gem_contents_spec.rb: loads the gemspec and fails if any packaged file falls outside that allowlist, or if a transcript, .local.json, or planning doc ever gets in. Runs under the default rake task, so CI is the pack-contents gate.

Before / after

• gem build on main failed on the 141-byte .architecture/ filename and would otherwise package 497 files, including .claude-sessions/ transcripts and .claude/settings.local.json (the published 0.2.0 already carries those).
• With this branch gem build succeeds and packages 119 files: lib/ plus exe/agentic, sig/agentic/builder.rbs, and the three top-level docs.

Verification

bundle exec rspec spec/gem_contents_spec.rb   # 3 examples, 0 failures
bundle exec standardrb agentic.gemspec spec/gem_contents_spec.rb
gem build agentic.gemspec                     # 119 files

The published 0.2.0 on rubygems still carries the stray files; yanking or cutting 0.2.1 is your call.

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