docs: fix CONTRIBUTE_COMPUTE.md calling a shipped feature 'the next step'

[wikithoughts]

What

`docs/CONTRIBUTE_COMPUTE.md`'s rate-limiting section closed with:

Rate limits bound abuse rate; gating who may register at all is the
next step — per-operator enrollment tokens.

This directly contradicted the same document, which earlier (join instructions
around what is now lines 55, 62-64, 82) already tells the reader to obtain and
use an enrollment token to join a coordinator.

Per D37 in docs/DECISIONS.md, per-operator enrollment
tokens shipped in Round 25 — they are not a "next step." With `PW_ENROLL` on:

• `/nodes/register` requires a valid `X-Enroll-Token` header
• `/admin/enroll` (operator-only, authenticated via the admin `PW_TOKEN`) mints new tokens
• `/users` (asker signup) stays open, but grants zero starter credit without a valid token

Change

Rewrote the closing parenthetical in the rate-limits (D36) section to describe
this shipped, present-tense mechanism instead of framing it as future work.
Skimmed the rest of the file for the same "next step"/"not yet" framing about
enrollment tokens — found no other instances.

Verification

• `ruff check .` — passed
• `pytest tests/ -q` — 507 passed, 1 skipped (docs-only change; sanity check)

No code changed — `docs/CONTRIBUTE_COMPUTE.md` only.

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