docs: idempotency contract + checkout dedup error envelope

[mastermanas805]

Summary

Backfills documentation for two in-flight InstaNode-dev/api PRs whose original briefs did not include a content-repo propagation step.

Pairs with InstaNode-dev/api#111 (server-side dedup on /api/v1/billing/checkout) and InstaNode-dev/api#112 (Idempotency middleware on all create endpoints with body-fingerprint fallback). Stacked on top of #5, which contributed the bulk of the idempotency contract documentation.

What this PR adds on top of #5

• A worked curl example in docs/idempotency.md showing two back-to-back /db/new calls from the same caller collapsing into one resource via the fingerprint fallback. Includes the exact X-Idempotency-Source / X-Idempotent-Replay headers an agent would see on each side, and a short callout that the same shape applies to /deploy/new (multipart) and /api/v1/billing/checkout.
• A POST /api/v1/billing/checkout concurrent-call dedup section in docs/limits.md with the full 409 checkout_in_flight envelope (retry_after_seconds: 60, agent_action copy, request_id), placed alongside the existing 400 tier_unavailable billing-error documentation. Documents the 60s TTL on the SETNX guard, that an Idempotency-Key is honoured on this route for a longer-window guarantee, and the fail-open posture on Redis brownouts.

Coverage map

Surface	State after this PR
llms.txt convention #7 (idempotency on by default)	already added in #5
docs/idempotency.md two-layer contract + headers + covered routes	already added in #5
docs/idempotency.md worked double-click curl example	added here
docs/idempotency.md callout that checkout has its own SETNX guard	added here
docs/limits.md 409 checkout_in_flight envelope	added here

Why one PR, not two

The user's rule is "every contract change propagates". Two parallel api PRs (#111 and #112) each introduce contract surface, and #5 already handles most of #112. Bundling the remaining bits in a single docs PR keeps the propagation traceable to a single landing event.

Notes

• Stacked on docs/idempotency-contract-2026-05-14 so the merge is clean once docs(api): document Idempotency-Key + fingerprint fallback #5 lands. After docs(api): document Idempotency-Key + fingerprint fallback #5 merges to main, this PR's diff against main will show only the 56 new lines added here.
• llms-full.txt is not in this repo today; the corresponding longer-form content lives in docs/idempotency.md and is mirrored by the existing .md route convention documented in llms.txt.
• No tests in this repo — docs/marketing copy only, no code change.

Test plan

• Build the Vite SSG against this branch and confirm /docs#idempotency renders the new "Worked example" subsection and /docs#limits renders the new "POST /api/v1/billing/checkout — concurrent-call dedup" subsection.
• Curl https://instanode.dev/docs/idempotency.md (after deploy) and confirm the worked example is present in the .md mirror.
• Confirm the JSON envelope in limits.md matches internal/handlers/billing.go in api PR #111 byte-for-byte (error, message, retry_after_seconds, agent_action).