fix(cli): make missing-companion-binary error actually helpful (#258)

@whereiszebra (issue #258) downloaded just \`deepseek-macos-arm64\` from
the GitHub Release, ran it, hit:

  error: deepseek-tui binary not found at /path/to/deepseek-tui.
  Build workspace default members to install it, or set DEEPSEEK_TUI_BIN
  to its absolute path.

…spent 11 minutes figuring out they also needed \`deepseek-tui-macos-arm64\`
sitting next to it, and self-closed with: "Release page does not document
that both deepseek-macos-arm64 and deepseek-tui-macos-arm64 must be
downloaded together."

The dispatcher's error was the wrong message for the population that hits
it most often — direct GitHub Release downloaders. "Build workspace default
members" is meaningless if you didn't clone the repo. \`DEEPSEEK_TUI_BIN\`
is also not what they need.

New message lists the three concrete install paths that actually work for a
fresh user — npm, cargo, or grab BOTH binaries from the same release
page — and keeps the env var override as a final fallback for power
users. No logic change; just better text. Existing
\`locate_sibling_tui_binary_honours_env_override\` test still passes.

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

crates/cli/src/lib.rs

@@ -1111,7 +1111,17 @@ fn locate_sibling_tui_binary() -> Result<PathBuf> {
     // expected name, not "deepseek-tui" on Windows.
     let expected = current.with_file_name(format!("deepseek-tui{}", std::env::consts::EXE_SUFFIX));
     bail!(
-        "deepseek-tui binary not found at {}. Build workspace default members to install it, or set DEEPSEEK_TUI_BIN to its absolute path.",
+        "Companion `deepseek-tui` binary not found at {}.\n\
+\n\
+The `deepseek` dispatcher delegates interactive sessions to a sibling \
+`deepseek-tui` binary. To fix this, install one of:\n\
+  • npm:    npm install -g deepseek-tui            (downloads both binaries)\n\
+  • cargo:  cargo install deepseek-tui-cli deepseek-tui --locked\n\
+  • GitHub Releases: download BOTH `deepseek-<platform>` AND \
+`deepseek-tui-<platform>` from https://github.com/Hmbown/DeepSeek-TUI/releases/latest \
+and place them in the same directory.\n\
+\n\
+Or set DEEPSEEK_TUI_BIN to the absolute path of an existing `deepseek-tui` binary.",
         expected.display()
     );
 }