fix: bring back three behaviors that only ever existed downstream

[MXAntian]

How these were found

Rebasing a downstream runtime onto main and diffing what the runtime had that main does not — not by grepping for the markers that were supposed to delimit the private parts. The marker grep said the port was complete. It was not, three times over.

The diff that actually worked was mechanical: every hasFlag('--x') branch, old vs new.

1. --record-conversation CLI branch (index.mjs)

The per-turn counterpart to scripts/transcript-sweep.mjs — a hook pipes one turn in on stdin as it happens, instead of the sweep catching it later.

recordConversationAsync() is exported; only the branch that reaches it was missing. Callers fall through to usage and exit 1, and turns silently stop entering the conversations table.

This branch has been lost to a refactor twice. The first loss ran 17 days before anyone connected the ~150 daily engram_failed warnings to it.

2. Session host binding check (mcp-server.mjs)

A session's host is bound at creation and every write on it is stamped with that host. If a later request on the same session presents a token mapping to a different host, the binding and the evidence disagree — exactly the case channel-derived provenance exists to prevent.

Reject under enforce; keep the binding but log loudly otherwise. The silent version is provenance drifting with no signal at all.

3. An empty .env.local value now clears the variable (mcp-server.mjs)

MNEME_QUARANTINE_HOSTS= exists to turn quarantine back off. Under "existing env wins" an inherited value survives that line: the file says off, the env says on, and writes keep landing in a table recall never reads.

"Existing env wins" is right for supplying a value and wrong for withdrawing one.

Verification

full suite            280 passed / 0 failed
--record-conversation recorded: 1  → row lands, non-ASCII content intact

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