ssh: import Fish and Nix profile paths

[0xSero]

Addresses the shell-environment portion of #185.

Root cause

Litter executes generated SSH commands under POSIX sh, but its profile importer only tried POSIX/bash/zsh files. A Fish login account—and common NixOS/profile locations—could therefore hide codex, Node, and package-manager binaries from the SSH command environment.

Change

• keep all generated commands under POSIX sh
• when Fish is the selected login shell, invoke fish --login --command only to export PATH through a sentinel-safe record
• ignore Fish startup chatter and fall back to the POSIX PATH if export fails
• restore common user/system Nix profile bin directories
• preserve existing POSIX/bash/zsh and Node-manager behavior

Verification

• /bin/sh -n
• shellcheck
• focused profile tests: 6/6
• SSH suite: 28 passed, 1 live-only ignored
• full codex-mobile-client lib suite: 778 passed, 0 failed, 5 live-only ignored
• git diff --check

Remaining acceptance

No real Fish/NixOS SSH host was available; Fish behavior was exercised through an executable fixture. Keep this draft until a live Fish host proves Codex/package-manager lookup. The reporter's folder-picker -32604 is a distinct app-server spawn path and is not claimed fixed without host diagnostics.

[0xSero]

The remaining Mobile CI run was intentionally canceled after shared-prep and Android passed, to prioritize #221's clean-runner test-hermeticity gate. #216 already has 778 green Rust tests, focused shell/SSH tests, and remains draft for a real Fish/NixOS host. Rerun the platform matrix with that live-host acceptance before readiness; this cancellation is scheduling, not a failure.