Built-in models now serve only agent runs

Self-hosting

• Enterprise: the temporary starter-credits bridge and its AGENTA_STARTER_CREDITS_BRIDGE_* settings are removed. New organizations get the wallet sign-up grant under the same sign-up policy as before.
• Enterprise: the migration ee0000000006 adds the account_tombstones table. An email that signs up again within AGENTA_ACCOUNT_TOMBSTONE_DAYS (default 365) gets no free credits.

Improvements2

• SecurityCode evaluators receive a scoped credential and no write-only provider keys
• SecurityOn Daytona, the gateway token stays out of the sandbox environment

Fixes2

• AgentsThe first run of a new agent that uses a stored Claude or ChatGPT sign-in no longer fails
• PiPi's ChatGPT subscription no longer offers models that ChatGPT refuses