docs: refresh README for B2B investors and partners

[kapustazh]

Summary

Refocus the README on OneShot's initial B2B buyer and the interrupted-payment
problem so investors and partners can understand the product and its stage.
Preserve setup, architecture diagrams, API routes and operational details in
docs/DEVELOPER_GUIDE.md, with corrected relative links.

Scope and acceptance criteria

• Documentation-only, focused README refresh.
• Explain agent builders as initial buyers, internal operators as prospective buyers, and service platforms as prospective partners.
• Separate prototype evidence from unvalidated demand and pricing.
• Add work@kapustazh.dev for feedback, integration and pilot enquiries.
• Preserve developer reference and testnet safety limits; no unrelated cleanup.

Product and security invariants

• Tenant isolation remains fail-closed.
• Sponsor authorization, auditability, and daily caps remain enforced where applicable.
• Recipients cannot modify sponsor controls or access sponsor-only data.
• No secrets or sensitive runtime configuration included.
• Non-applicable invariants explained.

Invariant notes: No runtime, policy, payment, contract, or deployment changes.
Existing payment boundaries and evidence limits remain explicit. The contact
address was supplied by the owner for public use. No new qualification claim.

Validation

Prettier --check README.md docs/DEVELOPER_GUIDE.md: PASS
Repository-configured markdownlint-cli2 on both documents: PASS
Local Markdown link existence: PASS
corepack pnpm format:check: PASS
corepack pnpm lint: PASS
corepack pnpm typecheck: PASS
git diff --cached --check: PASS

Local Node 22.23.2 differs from pinned Node 24.19.0; CI validates pinned runtime.
Behavior tests were not rerun locally for documentation-only edits.

Independent review evidence

Gate A — exact candidate tree before push

• Base commit SHA: a60836b
• Reviewer tool: free-pi-cli
• Verdict: SKIPPED at owner's explicit request: "we don't need freepi here. chill".
• No PASS claimed. Previous attempts returned no valid verdict.

Gate B — exact remote PR head

• Verdict: SKIPPED at the same explicit instruction.
• CI: pending at PR creation.
• Deliberate deviation from .agent/IMPLEMENTATION_LOOP.md; remains draft.

Risk and rollback

• Residual risks: customer demand and pricing remain unvalidated; README accurately labels prototype/testnet limits. Developer guide retains existing technical content, which can drift as implementation changes.
• Rollback: revert this documentation commit.

Human merge

• A human owner has reviewed the evidence and will perform the merge.