fix(tui): welcome splash, alt-screen, unavailable, mcp and plugins

[echobt]

Summary

Ships the Designer welcome lock inline (alternate screen never by default) on the same A–Z branch as the coding-service unavailable fix, MCP completeness, custom providers, and the plugin registry.

• Interactive TUI stays inline. alternate_screen defaults to false / never. The host shell prompt stays visible above the app. Cortex does not enter the alternate screen on start. Opt in only with [tui] alternate_screen = true or cortex --alternate-screen.
• First empty session splash: Welcome to **Cortex**, the coding agent CLI then v{package version} · / commands · @ files · ! shell · & cloud. Dual-hairline composer with the white block at input column 0 (caret before the placeholder). Footer remains model · mode · context | shift+tab.
• No mascot, mint, boxes, or painted fake shell lines. After the first user turn the splash is dropped; session empty is composer + footer only.
• Composer caret=0 empty rules from the prior lock are unchanged.
• Product-facing unavailable copy stays in this PR (no mock-success).
• MCP: cortex mcp tools, --header on mcp add, TUI loads servers from config.toml as well as local storage.
• Providers: create_client honors the configured provider base_url.
• Plugins: search / browse / update against CORTEX_PLUGIN_REGISTRY (default https://software.cortex.foundation/plugins). Unknown names no longer write a placeholder; install fails if the registry has no package.

Test plan

• Config / TuiConfig / TerminalOptions / AppRunner default alternate_screen = false (unit tests assert never)
• Welcome splash copy still covered by lock_proof / welcome card / empty-session harness
• Full CI green on 0403b30 (fmt, clippy, test, TUI checks, audit, version check)

Attestation (required)

I attest that:

• Security reviewed — auth, exec/sandbox, path traversal, network egress, and secret handling in this diff were reviewed. No secrets, tokens, or keyring dumps are in the change. MCP --header and bearer token env var names are documented as non-secret. Plugin registry errors do not leak HTTP client names.
• Product-facing errors — user-visible failures use Cortex product copy. API-down paths say The coding service is temporarily unavailable. Plugin registry failures say the registry is temporarily unavailable. No raw provider, SDK, or transport names.
• TUI verified — welcome, session empty, login selection wash, and composer lock are covered by headless lock_proof / welcome_card / login snapshot tests. Splash lock PNGs were recaptured on the welcome-copy revision; this change is default-inline only (no painted chrome change).
• Tests added — first-user-turn drops splash; welcome two-line copy; MCP tools command; plugin registry subcommands; default alt-screen is never. No mocks that report success.
• No secrets — no API keys, WorkOS secrets, R2/AWS credentials, or .env files are included.

Risk

Default TUI is inline (never alternate screen). Users who want a full-screen buffer must set [tui] alternate_screen = true. Plugin install no longer creates a fake placeholder for unknown names.