docs: document /coven substrate surface and TUI delegation

Adds a Recent Highlights bullet pointing users at /coven and the
coven-cli → coven-code exec handoff (with the COVEN_LEGACY_TUI
opt-out documented).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: BunsDev

README.md

@@ -22,6 +22,7 @@ Recent highlights:
 - **/share** — share sessions via unlisted GitHub Gists `[EXPERIMENTAL]`
 - **Free Mode** — try `/connect` for a free-tier agentic coding experience `[EXPERIMENTAL]`
 - **/goal** — `/goal <objective>` keeps the agent working across multiple turns `[EXPERIMENTAL]`
+- **/coven** — drive the local [Coven daemon](https://github.com/OpenCoven/coven) (sessions, harness runs, rituals) without leaving the TUI. `/coven` is the unified replacement for `coven-cli`'s interactive menu and the legacy `coven-tui` slash shell; when `coven-code` is on `PATH`, `coven` and `coven tui` exec into it automatically (opt out with `COVEN_LEGACY_TUI=1`). Run `/coven help` for the subcommand list.
 
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