fix(session): default an unstated spawn cwd to the user's home

[karngyan]

The bug

A session spawned with no cwd opens at / on service-managed machines. Registry.Spawn handed opts.Cwd straight to cmd.Dir, so an empty cwd — what the UI's plain New-session button sends — made the shell inherit the daemon's own working directory. A daemon under launchd or systemd runs at / (neither unit in internal/service/unit.go sets a working directory), so every default session landed there; a daemon started from a terminal put them wherever it happened to be launched from, which is no better chosen.

The fix

In Registry.start, an empty opts.Cwd now resolves to os.UserHomeDir() — the default every terminal emulator and sshd picks, and the same call Revive already uses for a snapshot whose directory has vanished. The one resolved value feeds both cmd.Dir and the recorded Info.Cwd; the old code seeded the record from os.Getwd() instead, a recorded-vs-actual split this closes. When even home is unresolvable the empty string stands and the shell inherits — the old behaviour, kept as the floor rather than the default. ($HOME is set by launchd user agents and systemd user managers alike, so the resolution works on exactly the path the bug describes; unlike $SHELL, no user-database fallback is needed.)

Callers that state a cwd are untouched — they all pass a non-empty value:

• group-heading spawns (spawnFromGroup → client.spawn({ cwd }))
• restart with the exited session's cwd (terminal.tsx handleRestart)
• revival from snapshot (Registry.Revive passes snap.Cwd, or home when it has vanished)

Tests

Two new tests in internal/session/registry_test.go, following cwd_test.go's poll-and-resolve pattern against the real child process:

• TestSpawnDefaultsCwdToHome — empty cwd: the recorded Info.Cwd is $HOME and processCwd confirms the child actually sits there.
• TestSpawnKeepsExplicitCwd — stated cwd (deliberately not $HOME): recorded and actual both stay put.

Evidence: make web relay then go test ./... — all packages ok (including internal/session 7.5s) — and go vet ./... clean.

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