fix: pi extension banks a session id only once its file exists

[scgopi]

Refs #214. Found while reviewing #338.

Problem

pi picks a session id at startup, but it only writes the session file when the first assistant message arrives. The presence extension banked the id at session_start, before that file existed. pi --session <id> exits 1 with No session found matching '<id>' when an id has no file. So the banked id was dead whenever:

• a pi session was quit before its first reply (e.g. a turn-based loop opened with no prompt), or
• a reboot interrupted a /new before its first reply. The dead id replaced the id of the conversation before it, so that conversation could no longer be resumed.

Locally the ensure and the pane notice the resume dying and fall back to a fresh launch. That fallback costs an extra launch and a resume-dead entry in the dial log, and it can't recover the conversation the dead id replaced.

Fix

bank() now writes the id only once getSessionFile() exists on disk. It retries at tool_call and agent_settled, the first points where pi has saved an assistant message. A session that already has a file, such as a resumed one, still banks at session_start.

Verified against pi 0.85.1

Tested with the real pi binary, a fake zmx shim, and the rendered extension:

Launch	Before	After
Bare TUI, killed before any reply	id banked, no session file, --session exits 1	nothing banked
TUI with a prompt (the provider error is saved as an assistant message)	banked, file exists	banked, file exists

A new test, anIDIsBankedOnlyOnceItsSessionFileExists, checks that the file-existence check is in place and that both retry handlers call bank().

🤖 Generated with Claude Code

[scgopi]

Gate on 83131e3a (= remote head), private DerivedData: xcodebuild test exit 0 — 1732 tests / 183 suites / 0 failures (+1 over the #338 base); graphcode-cli build exit 0; graphcoded build exit 0; swiftlint 0 errors; swift-format clean; cli-smoke exit 0; Linux pass.

Fails without the fix: with the base PiPresenceExtension.swift checked out, anIDIsBankedOnlyOnceItsSessionFileExists records 3 issues (xcodebuild exit 65).