feat(tui): grid layout + auto-growing scroll input

[gemone]

What

Redesign the TUI layout for streaming responsiveness — fully layout-driven, no dock hacks.

Layout: dock stacking → Grid

The old CSS mixed layout: vertical with height: 1fr and two dock: bottom widgets (input, footer). Replaced with a four-row Grid:

Screen { grid-size: 1 4; grid-rows: 1 1fr auto 1; grid-columns: 1fr; }
Header { dock: none; }                 # row 1 — title bar opts into the grid
#main (Horizontal)                     # row 2 — 1fr conversation + sidebar
TextArea#input { height: auto; ... }   # row 3 — auto-grow scroll input
CothisFooter#footer                    # row 4 — status bar

Every pane is placed by the grid; the footer's dock: bottom is removed from its own CSS (the grid owns placement).

Input: auto-growing scroll input

The input TextArea grows with content (min-height: 3, max-height: 8) then scrolls internally past the cap, so a long prompt never squeezes the conversation out of its 1fr row. Single input box; Ctrl+Enter send and /-command semantics unchanged.

Streaming path

Untouched — #407 (plain Static during the stream, one Markdown parse at finalisation, 50 ms refresh throttle) is already the optimal shape. The responsiveness win is the layout: the conversation gets the full remaining height.

Tooling: ty venv resolution

Adds [tool.ty.environment] python = ".venv" to the root pyproject.toml. The LSP auxiliary ty scanner launched ty server from a cwd where .venv auto-discovery missed, so it reported false unresolved-import findings for every third-party and first-party module (the real ty check was already clean). The directory form is portable across OSes — ty locates bin/python / Scripts/python.exe inside the venv itself.

Changes

• packages/cothis-tui/src/cothis/tui.py: Grid CSS, Header { dock: none }, input auto-grow CSS, footer dock removal, docstring updated for the four-row layout.
• pyproject.toml: [tool.ty.environment] python = ".venv".
• tests/test_tui.py: two new layout-contract tests.
• tests/test_tui_pty.py: docstring updated for the four-row grid; # noqa: B009 on the deliberate Windows-ty-safe getattr pattern.

Problem fixed

• The pane layout was assembled from mixed layout modes + docks, producing an awkward arrangement where the conversation did not reliably own the remaining space.
• The input box was a fixed 3 rows — a long prompt squeezed the conversation; it now grows and scrolls internally.
• The LSP auxiliary ty scanner reported false unresolved-import errors because its ty server could not resolve the project venv.

Tests

• test_grid_layout_stacks_panes_vertically — header < main < input < footer vertical order; conversation taller than input + footer; full width in single-session mode.
• test_input_auto_grows_then_scrolls_internally — input grows past the min-height floor, caps at max-height, scrolls internally; conversation shrinks accordingly.

101 TUI tests green; ruff check + ty check clean (venv, from any cwd). CI green on all three OSes.

Pre-existing, not from this PR

test_startup_latency.py::test_cothis_help_under_startup_budget fails on this machine (1156 ms vs 1100 ms ceiling) — it reproduces on a clean checkout of origin/main (stash-verified). This PR touches no CLI startup path; CI runners pass it.