docs: sync main README with develop

[kapustazh]

Summary

Make main's README match develop exactly, including the linked developer guide.
Source: develop at 55a387a.
No other develop application changes are included.

Scope and acceptance criteria

• README and developer guide match source byte for byte.
• Local relative links resolve; repository-wide Markdown lint passes.
• 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 below.

Invariant notes: documentation copy only; no application behavior or new qualification claims.

Validation

Exact develop file equality: PASS
Local relative Markdown links: PASS
Pinned repository-wide markdownlint-cli2: PASS
git diff --check: PASS

Independent review evidence

Gate A — exact candidate tree before push

• Verdict: SKIPPED per user's explicit instruction to omit FreePi for this work.
• No PASS claimed.

Gate B — exact remote PR head

• Verdict: SKIPPED per the same instruction.
• CI pending at creation; draft PR.

Risk and rollback

• Residual risks: copied documentation describes develop; main code may differ.
• Rollback: revert this documentation commit.

Human merge

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