docs(tui): add tui lock v2 design pack (boards + spec)

[echobt]

Summary

Design-only deliverable for the full TUI redesign — no runtime code changes. Adds docs/media/tui-lock-v2/:

• 108 lock boards (77 at 120×40, 31 at 40×12) rendered in IBM Plex Mono on the inky Cortex chrome: welcome (cortex / agent), session (user prompt bars, ♦ Thought for Xs, replies, Worked for Xs, opt-in banner, ▼), dual-hairline composer with mode chip (top-left) and model chip (bottom-right), contextual footer shortcut strip, token counter top-right, slash palette with violet fuzzy matches, /model list + High/Medium/Low effort radios, Settings modal (search, categorised rows, tip + nav legend, theme submenu), modes (Agent/Plan/Ask/Bash), approval/confirm/question radios, MCP/plugins/usage/quota/sandbox/cloud panels, diagnostics (red/amber), stopped, product-facing API error, tool tiles, diff hunk, markdown table, code fence, login flow, shortcuts overlay, resume/jobs/skills/files pickers, todos, sudo, config, btw, compact mode.
• Hover ≠ focus on every interactive surface: hover = #1A1A1A bar / #525252 hairline, no violet; keyboard focus = #262626 bar + violet #A78BFA marker.
• SPEC.md — tokens (JSON), viewport geometry, component recipes, hover/focus matrix, welcome copy for both entrypoints, reference→Cortex rename table, state checklist, keybinding contract, and 12 flags for decisions the design could not make safely.
• index.md — generated board index; txt/<size>/<board>.txt — exact character grid of every board (expected buffers for later snapshot tests).
• tools/render_lock_v2.py + tools/boards.py — Pillow cell-grid renderer that regenerates the pack (python3 tools/render_lock_v2.py --index).

Audits run on the pack: zero competitor/provider strings in pixels, grids, spec or tooling; green only on ✓/+, red/amber only on errors/warnings, violet only on keyboard focus and typed slash matches.

Test plan

• Docs-only change under docs/media/tui-lock-v2/** — no Rust touched, so cargo fmt / clippy / cargo test / cargo audit are unaffected (not run).
• Pack regenerated from the committed tooling; string audit (rg -i for competitor names) and palette audit (exact-hex presence per board) pass.
• TUI / snapshot tests — n/a (no TUI surface changed in this PR; the txt/ grids are the expected buffers for the implementation PR).

Attestation (required)

I attest that:

• Security reviewed — no auth, exec/sandbox, network or secret handling in this diff (documentation and PNG/TXT assets only).
• Product-facing errors — boards use Cortex product copy only; the API-down board reads The coding service is temporarily unavailable. No provider, SDK or transport names anywhere in the pack.
• TUI verified — n/a for runtime; every board was rendered and visually reviewed at both sizes.
• Tests added — n/a (no code under test); grids are provided as future snapshot fixtures.
• No secrets — none included.

Risk

None at runtime. Review focus: the 12 flags in SPEC.md §9 (notably: agent entrypoint does not exist yet; Ctrl+x is drawn as shortcuts but currently clears the follow-up queue; Mouse settings need new tui.mouse.* config keys).