docs: add AI agent cookbook

[vku2018]

What & why

Closes #12.

Adds a worked AI-agent cookbook for the tributary CLI:

• setup commands for finding a studio/session/recording and transcribing it
• a filler-word removal recipe with an agent prompt, word JSON example, padded cut ranges, and export commands
• a three-clip vertical highlight recipe with transcript commands and --aspect 9:16 --captions renders
• a show-notes/chapters recipe with transcript commands and a Markdown output shape
• a link from the existing docs/AI-AGENTS.md quick reference

How I verified it

• pnpm typecheck passes
• I drove the affected flow in a browser and it behaves as described
• N/A - no recording-path code changes

Validation run locally:

• git diff --check
• node cli/tributary.mjs help
• CI=true corepack pnpm@10 install --frozen-lockfile
• corepack pnpm@10 typecheck

This is documentation-only, so I did not drive a browser recording flow.