Start a session in the user's home, from one place

[gmpassos]

A shell opened wherever the node process happened to be standing. For an agent
installed as a service that is where its binary lives — /usr/local/bin — which
is nobody's idea of a starting point.

The behaviour existed, in the wrong place

omnyshell node start already passed workingDirectory: home
(bin/omnyshell.dart), but it did so in the CLI rather than in the backend.
An embedder building its own ProcessShellBackend — as OmnyServer does —
got neither that nor anything else:

omnyshell.ProcessShellBackend()     // no workingDirectory

A default that only one caller applies is not a default.

The fix

resolveStartDirectory now holds the decision, in the order of who is entitled
to make it:

1. The client's request — ~ expanded; a directory that does not exist
   still fails loudly, which is the right answer to an explicit request that
   cannot be honoured.
2. The node's configuration — for an embedder with an opinion.
3. The user's home — resolved through existingUserHome, so it works on a
   service-run node that was handed no HOME (1.57.1).
4. null — inherit the node's own directory, as before.

exec follows the same rule as an interactive shell, so the two agree; ssh
set that expectation long ago.

A home that does not exist is skipped rather than used: handing a missing
path to Process.start fails the session outright, which is worse than opening
somewhere unremarkable. A blank string falls through for the same reason.

Drift this turned up

Consolidating the three backends onto one function exposed that
ScriptPtyShellBackend — the default on Linux and macOS — never expanded a
leading ~. So --cwd ~/project worked on the pipe and winpty backends and
silently did not on the one nearly everyone uses. They share the resolution now
and cannot diverge again.

The Windows MSYS path translation was duplicated in two backends; it is applied
once, where the decision is made.

_resolveNodeHome keeps only what is particular to it — the node profile's
HOME overriding the process environment — and defers the rest to
existingUserHome, so the password-database lookup a service-run node needs is
not reimplemented beside it.

Verified

The new backend test builds a bare ProcessShellBackend() from a process whose
cwd is the repo, and both an exec and an interactive session report the home
directory — the embedder shape that had nothing before. Unit tests cover the
precedence chain, the blank-string fall-through, the ~ expansion and the
refusal to use a home that is not on disk.

696 unit and 136 integration tests pass; dart analyze clean.

v1.57.1 is tagged, so this carries the bump to 1.57.2 — pubspec.yaml and
lib/src/version.dart, pinned together by version_test.dart — plus its
changelog section.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KdG8bDrddXkEkzgPq23D6q

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...frastructure/backend/pty/winpty_shell_backend.dart	0.00%	3 Missing ⚠️
...b/src/infrastructure/backend/shell_invocation.dart	83.33%	1 Missing ⚠️

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