docs: document agentic authoring workflow in README

[jdwit]

What

Documents the agentic, transcript-grounded, on-brand metadata authoring workflow in the README:

• New Agentic authoring section explaining the design: the CLI never calls a model itself, it supplies the raw materials (caption tracks, clean transcript, per-channel brand voice) and applies the drafted metadata through the ordinary videos update path.
• Feature bullet added to the overview, pointing at the new section.
• Reframes earlier "an agent (or you)" wording into a dedicated section so the transcript-as-source-of-truth and brand-file-as-tone design is explicit.

Pure docs; no code or behaviour change. The underlying feature landed earlier (#52, #53, #54).

Closes #43

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.03%. Comparing base (582d41b) to head (d50fbb7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   85.03%   85.03%           
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  Files          15       15           
  Lines        2660     2660           
  Branches      393      393           
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  Hits         2262     2262           
  Misses        264      264           
  Partials      134      134           

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