feat: show startup progress in the chat feed

[wowi42]

Summary

Launching zerostack dropped the user into an empty feed — no logo, no indication of what was loaded, and MCP servers connecting silently in the background. This PR makes the startup sequence report itself directly in the chat UI, where it stays as scrollable history (no separate splash screen, single alt-screen entry).

zerostack v1.7.2

Ready to code; type a request or '/' for commands

… Loading 3 MCP server(s)
✓ MCP Exa Web Search — 2 tools (577ms)
✓ MCP glitchtip — 5 tools (1.8s)
✓ MCP redmine — 12 tools (930ms)
✓ agent ready — 3 MCP server(s), 19 tools connected

Changes

• Banner: plain zerostack vX.Y.Z line — no logo, in keeping with the IBM-style design. Model/context/prompt/branch/cwd are not repeated — they already live in the statusline.
• Startup log (new src/ui/boot.rs, pure + unit-tested), opt-in via show_additional_info_startup (default false): global/local config, prompts/themes with project-local counts, context files, compile-time feature list, session restore, subagents, model info, tools & permissions — with per-step timings.
• Live MCP loading: servers connect in the open chat UI via connect_all_with_progress, one line per server with tool count and connect time; McpClientManager caches tool_counts and the agent-ready line totals them.
• Surfaced warnings: subagent provider fallback, advisor disabled, and missing sandbox backend were previously only in tracing logs — they now show as red ! lines.
• Bug fix: draw_bottom panicked with u16 underflow ("attempt to subtract with overflow") on tiny/0-height terminals; input and cursor rows now reuse the already-saturating input_top.
• Config: new show_additional_info_startup key (default false) to opt into the startup info lines; MCP loading lines always show. Documented in docs/CONFIG.md.
• Headless paths (-p, --loop, --acp) are untouched — stdout stays clean.

Verification

• cargo test: 706 passed (incl. new tests for boot state, banner, feed color override, durations, features list)
• cargo fmt, cargo install --path . --debug
• Smoke-tested on a real pty: single alt-screen entry, correct ordering/colors, timings, tool counts, no panic on 0×0 terminals

[gi-dellav]

make sense, but:

1. I think that the gradient logo is a little bit excessive, break the IBM design that inspired the original zerostack TUI, and makes us the same as all other agents

2. While showing MCP is really useful, all other informations is useful only for a small subset of users, as it was never requested; I think it should be moved to an opt-in config flag called "show_additional_info_startup" or something similar

[wowi42]

Thanks for the review — both points addressed in the merge commit:

1. Logo removed. The ASCII/gradient banner is gone; the banner is a plain zerostack vX.Y.Z line again, in keeping with the IBM-style design.
2. Opt-in startup info. All the non-MCP startup lines (config paths, prompts/themes, features, step timings, etc.) are now behind a new show_additional_info_startup config key, default false. Only the MCP loading lines (… Loading N MCP server(s), ✓ MCP <name> — N tools (t), ✓ agent ready) show by default, since those are the genuinely useful ones.

Also fixed the CI failures:

• clippy --all-features: boxed PrebuildEvent::Done (large_enum_variant), allowed the cfg-gated pushes in active_features (vec_init_then_push).
• clippy/test --no-default-features: irrefutable if let and never_loop in the no-mcp resolve_prebuild, unused mut / dead code under --no-default-features.

All four flavors now pass locally: cargo clippy --all-features -- -D warnings, cargo clippy --no-default-features -- -D warnings, cargo test (731), cargo test --all-features (952), cargo test --no-default-features (616).