docs: correct .env.example's reticulum node id derivation (RE8)

[eknuth]

.env.example still described the #896 model — the ingestor owning an RNS identity and deriving its !xxxxxxxx from "the transport identity already in RETICULUM_CONFIG_DIR".

#897 shipped the opposite. SPEC RE8 excludes the transport identity from the pick, because RNS generates it as an independent keypair that matches none of the operator's announced destinations — keying on it registered !fbf8e338 for a host whose primary identity was 27716218…. The id is the discovered primary identity, the one announcing the most destinations on the machine.

README.md says this correctly ("It is not the transport identity, and not the hash rnstatus prints as 'Transport Instance'"). .env.example contradicted it in both places it mentions the derivation, and an operator setting the variable reads .env.example, not the README.

Also carries over the strands the old row caveat #897 added to README.md and CHANGELOG.md, since .env.example is where an operator is standing when they set the variable that moves the id.

Docs only — no behavior change.

Tests

Two guards added to TestReticulumDeploymentSurface, both negative-tested against main where they correctly fail:

• test_docs_do_not_key_the_ingestor_on_the_transport_identity — README and .env.example must not derive the id "from the transport identity", and must name the primary identity.
• test_docs_warn_that_changing_the_id_strands_the_old_row

$ python -m pytest -q
1443 passed, 1 warning in 35.05s

$ black --check tests/ data/
All done! ✨ 🍰 ✨
91 files would be left unchanged.

https://claude.ai/code/session_01F8MHsE7xT4BhcQxPJf2AUR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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