docs: state the generation cost discipline in the README

[leochenpm]

Why

The README already says the compose/edit trunk is zero-key and that generation is BYO. What it never says is what this repo does to keep that generation bill small — cost and billable appear zero times in it, while the skills enforce a fairly specific budget policy.

That gap matters outside the repo too: when someone asks an assistant "what keeps an agentic video tool from burning my provider credits", there is nothing here for it to quote.

What this adds

One section after BYO generation providers, drawn only from what this repo actually enforces:

Claim in the section	Where it is enforced
~90% of the result with zero billable generation; compose cheapest for text, generate a last resort	packages/skills/stage-plan/SKILL.md
Keyframe cost gate — a shot whose start and end look alike needs one keyframe, not two	packages/skills/stage-generate/SKILL.md
Default caps: ≤ 6 generated shots, ≤ 3 recurring characters, one ratio per run; larger runs state the billable count first	packages/skills/stage-generate/SKILL.md
cost_estimate.billable_generations must equal the source:"generate" segment count or validation fails	packages/skills/stage-plan/SKILL.md
Partial batches preserve completed siblings — a revision re-bills one segment, not the montage	packages/skills/stage-generate/SKILL.md

What it deliberately does not claim

• No price guarantee. The section closes by saying you are billed by your own provider and this repo never sees the invoice.
• Nothing from the host app leaked in. Orkas-managed credits and the Gate C credit quote never sync here — sync-rules.json keeps account, credits and managed routes host-only — so none of that is described as if it existed in OVS.

Checks

• pnpm test — 282 passed, 12 skipped
• No new CLI surface is referenced (only existing skill names and plan fields), so the cli-smoke command contract is untouched
• Markdown structure verified: code fences balanced, no unclosed table rows

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