fix(pi): persist migrated pi() session entries

[eersnington]

A session stored by an older pi-coding-agent format is migrated in memory on every load, and its rows stay in the old format. This rewrites them once.

	Before	After
Load a session in an older format	SessionManager migrates it in memory, on every load	rewritten once, in one transaction
Stored pi_entry rows and header version	old format	current format

if ((stored.header.version ?? 1) >= CURRENT_SESSION_VERSION) return stored;
const entries = structuredClone(toFileEntries(stored));
migrateSessionEntries(entries); // Pi's own migration
// rewrite pi_entry and pi_session.header_json in one transaction

• Uses Pi's own migrateSessionEntries. No migration of our own.
• Stored sessions stay on the format the running pi-coding-agent writes, so a session format change upgrades every actor's storage once, on its next wake.

---

This is part 11 of 11 in a stack:

•  11  fix(pi): persist migrated pi() session entries #330 👈
•  10  feat(pi): expose the pi session to actor code as c.pi #329
•  9  perf(pi): connect the pi() sandbox on first use #328
•  8  feat(pi): drain and resume pi() runs #327
•  7  chore(examples): bump the docs examples to pi-coding-agent 1.0.2 #326
•  6  test(pi): let a run finish and the actor sleep when its caller disconnects #325
•  5  feat(pi): drain pi durable actors across upgrades #324
•  4  feat(pi): stream pi durable conversations to clients #323
•  3  feat(pi): add piDurable actor #322
•  2  feat(pi): store pi durable sessions in actor sqlite #321
•  1  feat(pi): add piDurable() and drain, resume, and c.pi for pi() #320

[eersnington]

send it