feat: carry pi sessions through node export/import

[scgopi]

Builds on #338. An exported pi loop now carries its conversation, and the imported loop resumes it instead of starting fresh.

What changed

• Export: the pi session file (~/.pi/agent/sessions/<cwd slug>/<timestamp>_<id>.jsonl) travels with the loop. Local loops find it by the banked id. Remote loops fetch it over the same tar-over-ssh path Claude sessions use.
• Import: the session's header gets a fresh id and the target working directory. The file is installed under the target's own slug, and the fresh id is banked so pi --session <id> resumes it. This works for both local and remote projects.
• The slug has to be the target's own. pi only offers an interactive "fork into current directory?" prompt for a session it finds under another project, and that would stall a loop.
• Layout and file helpers moved into SessionTransplant+Layouts.swift. The enum was already over swiftlint's type_body_length limit on the base branch.

Verified against pi 0.85.1

• A session file shaped the way restore writes it (fresh id, rewritten header, target slug) is listed and opened by pi's own SessionManager under a throwaway HOME, with the right id, cwd and entries.
• pi --session <that id> gets past session lookup to the model call. An unknown id prints No session found.

Tests

• PiSessionTransplantTests: header rewrite, refusal of files that aren't pi sessions, slug encoding, filename shape, and the remote install script run through /bin/sh under a fake HOME (lands at the Swift-computed slug, banks the id).
• The remote export and install suites now cover pi.

Refs #214

[scgopi]

Gate on 95471b86 (= remote head), private DerivedData: xcodebuild test exit 0 — ✔ Test run with 1739 tests in 184 suites passed after 83.905 seconds. / ** TEST SUCCEEDED **, all 8 new pi tests passed; graphcode-cli build exit 0; graphcoded build exit 0; SwiftPM build exit 0 + CLI smoke exit 0 (5 commands); swiftlint 0 errors; swift-format clean; Linux pass (https://github.com/scgopi/GraphCode/actions/runs/34724108071/job/103635100484).