Rename POPULACE_LEDGER_* client env vars to POPULACE_LOGBOOK_*

Small follow-up from the #628/#632 Logbook rename: the remote client's environment contract still uses the pre-rename names — POPULACE_LEDGER_URL, POPULACE_LEDGER_KEY, POPULACE_LEDGER_API_KEY (packages/populace-build/src/populace/build/logbook.py, _remote_config).

"Ledger" is exactly the word the naming split vacated (the data platform's Ledger is now PolicyEngine/chronicle), so these names will mislead operators. Rename to POPULACE_LOGBOOK_URL / POPULACE_LOGBOOK_KEY / POPULACE_LOGBOOK_API_KEY with a one-release back-compat window (read old names, warn once), then drop.

Note for whoever picks this up: the operator keychain currently stores the credentials under the OLD names; flip those in the same change. Coordinate with the populace→microcosm namespace sweep if it's still in flight — this is three constants and a docstring, so rebasing it is trivial.

🤖 Generated with Claude Code

Start in packages/populace-build/src/populace/build/logbook.py at _remote_config and trace where the operator keychain stores these credentials. Check the related #628/#632 rename work and any namespace sweep before changing the three constants and docstring. Done means the LOGBOOK names are primary, old names are read with a one-time warning for one release, and the keychain uses the new names.