fix(tui): allow overriding new session directory

[Sayt-0]

Summary

• accept an optional working directory with /new [dir] in the full TUI
• resolve relative paths from the active session and expand home and environment references
• preserve the --working-dir default for bare /new, Ctrl+T, and new-tab buttons
• validate explicit directories before spawning a session
• document the new override and --working-dir behavior

Closes #4046

Issue expectations

Expectation	Implementation
Keep --working-dir as the default for generic new-session actions	Bare /new, Ctrl+T, and both + buttons continue to reuse it
Allow a session to use a different directory without restarting	/new <dir> explicitly overrides the default for one session
Preserve the picker when no default is configured	Bare /new still opens the picker without an explicit --working-dir
Update the CLI documentation	Both --working-dir entries and the TUI slash-command reference are updated

Design notes

• The command override proposed in the issue was selected instead of adding a modifier binding.
• Drive-relative Windows paths such as C:foo retain Go's standard filepath behavior; ordinary absolute and relative Windows paths are supported.

Validation

• task test
• task build
• task check-plan-cross
• golangci-lint run with v2.13.1 built using Go 1.27
• go run ./lint .
• go mod tidy --diff
• npx --yes markdownlint-cli2@0.22.1
• ./scripts/docs-check-canonical.sh