refactor(pi): move databaseEnv into pi and version pi_file

[eersnington]

databaseEnv stores files of an agent without a sandbox in the Actor's SQLite. It created its pi_file table on first use, outside migratePiTables, so a later column change had no schema version to migrate from. The env has nothing to do with sandboxes, and pi() is its only user.

 packages/
 ├── pi/src/
+│   ├── database-env.ts   # moved; takes PiDatabase
 │   └── storage.ts        # migration 3: CREATE TABLE IF NOT EXISTS pi_file
 └── sandbox-adapter/src/pi/
-    ├── database-env.ts   # ensureTable, WeakMap cache, FileDatabase
     └── sandbox-env.ts

• ensureTable, its per-database WeakMap, and the table check before every operation are gone.
• IF NOT EXISTS keeps agents that created pi_file on 0.5.1 working with their files.
• @rivet-dev/sandbox-adapter/pi no longer exports databaseEnv or FileDatabase. No docs or examples use them.

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This is part 4 of 9 in a stack:

•  9  docs(agents): type the React example's root id as ConversationId #346
•  8  feat(pi): prefix Pi Durable's tables in actor sqlite #345
•  7  chore(pi): bump and test Pi Durable 1.1.0 #344
•  6  refactor(pi): type action ids with Pi's branded id types #343
•  5  refactor(sandbox-adapter): simplify sandboxEnv #342
•  4  refactor(pi): move databaseEnv into pi and version pi_file #341 👈
•  3  fix(sandbox-adapter): stop waiting on aborted Daytona exec #340
•  2  fix(sandbox-adapter): stop Pi env calls when the run aborts #339
•  1  fix(pi): read an app credential with one source call #338