fix(tui): resolve checkout from any folder + back-compat [tui] extra

[akozak-gd]

Problem

Two issues with the installed specflow CLI / TUI:

1. uv tool install --editable "./mcp_server[tui]" warned does not have an extra named 'tui'. textual had been moved to a base dependency, which deleted the tui extra entirely, while older install commands still referenced [tui].
2. specflow tui only launched from inside the cloned repo. Run from any other folder it flashed the app open and exited with no visible message (the in-app gate could only notify() then exit(), which tears the screen down before the toast paints).

Fix

[tui] extra — textual is a base dependency, so a fresh uv tool install --editable ./mcp_server (as documented in QUICKSTART) always has a working TUI. The now-unused tui optional-dependency extra is removed; the only references left were the extra itself and a stale test comment.

Resolve the checkout from any folder — the documented install is uv tool install --editable ./mcp_server, so the binary runs the code straight from the clone. Added installed_repo_root() (derives the checkout from the module's own __file__) and resolve_repo_root() (cwd walk-up, else the install's own checkout), factored on a shared _find_sentinel_root helper. run_tui and specflow init now use resolve_repo_root, so they work from any directory with no extra setup — running from inside a different checkout still wins.

Graceful failure — when no checkout can be resolved (non-editable / PyPI install with no clone), the TUI prints an actionable, audience-aware message instead of silently flashing open:

• self-hosting → run from the clone, pass --root-path, or install editable;
• remote backend → the TUI doesn't apply, use the MCP tools / CLI.

Notes / scope

• Making the stack run with no clone at all is intentionally out of scope: the backend image is built locally from ./backend/Dockerfile (no published image) and compose bind-mounts ./scripts, ./workspaces, ./agent_logs — the clone is the runtime, not just config. This change only fixes where the TUI finds the checkout.
• Zero-setup "works from any folder" is inherent to the editable install. A PyPI install resolves the checkout via cwd / --root-path only, and self-host self-heals because specflow init reinstalls the tool editable.

Tests

• test_local_env.py — installed_repo_root / resolve_repo_root (cwd-wins, fallback, none-when-neither).
• test_tui_app.py — launches from a resolved checkout; refuses visibly when none resolves.
• test_cli_init.py — updated no-checkout case.
• Full mcp_server suite passing.

Verification

• uv tool install --reinstall --editable . → no warning, textual installed, TUI tests pass.
• cd /tmp && specflow tui → resolves the clone and proceeds (no "must be run from checkout" error).
• cd ~ && specflow init → runs against the same clone.