docs: refresh README and configurations for shipped features

[quiet-node]

Summary

Bring README.md and docs/configurations.md back in sync with everything shipped on main since the last README touch (v0.5.0 → present).

README.md

• Features: add agentic-search (/search), in-app model picker, cross-model continuity, four-tab Settings panel (⌘,), and the contextual tip bar.
• Install: link the new nightly release channel alongside the latest stable DMG.
• Configuration: replace the stale "(quote display limits and system prompt)" one-liner with a current schema overview covering [inference], [prompt], [window], [quote], [search], plus a pointer to tuning-context-window.md.
• What's next: remove "Internet search" (shipped via /search) and "Native settings panel" (shipped in feat(settings): add four-tab settings panel with Tauri config read/write commands #108); trim the redundant live-slash-commands list.

docs/configurations.md

• Replace the "A future Settings panel will let you make the same changes from inside the app" sentence with current-tense language now that the panel is live.

No code changes; docs-only.

Test plan

• Skim rendered README on the PR page for layout/links.
• Confirm nightly link resolves (https://github.com/quiet-node/thuki/releases/tag/nightly).
• Confirm Configuration section accurately reflects the current TOML sections.