docs: align README install section with the landing page (#137)

* docs: align README install section with the landing page

Lead with the install.sh one-liner (it also sets up the odek engine),
note bodek upgrade, then Windows/manual releases, then source build.

* docs: correct installer claims in the README install section

The installer offers to install the odek engine (piped installs default
to No; --with-odek to force) and prefers /usr/local/bin with a
~/.local/bin fallback.

Author: jkyberneees

README.md

@@ -65,26 +65,24 @@ bodek reuses that exact protocol from the terminal, which means:
 
 ## Install
 
-**Prerequisite:** bodek is only the front-end — you also need the `odek`
-engine. See [odek's install instructions](https://github.com/BackendStack21/odek)
-(provider env key such as `DEEPSEEK_API_KEY` / `ZAI_API_KEY` — see odek's [PROVIDERS.md](https://github.com/BackendStack21/odek/blob/main/docs/PROVIDERS.md)).
-
-### Prebuilt binaries
-
-Download the latest compiled binary from the
-[releases page](https://github.com/BackendStack21/bodek/releases) — archives
-are published for Linux, macOS, and Windows (amd64 & arm64), with
-`checksums.txt` for verification.
-
-One-liner for Linux / macOS (checksum-verified, installs into `~/.local/bin`):
-
+One-liner for macOS & Linux (checksum-verified, installs into `/usr/local/bin`
+or `~/.local/bin` when that is not writable).
+It offers to set up the `odek` engine, which bodek needs on `PATH` (piped
+installs default to No — rerun with `--with-odek` to install it
+unconditionally):
 
 ```bash
 curl -fsSL https://bodek.21no.de/install.sh | sh
 ```
 
+You still need a provider API key for the engine (`DEEPSEEK_API_KEY` /
+`ZAI_API_KEY` — see odek's [PROVIDERS.md](https://github.com/BackendStack21/odek/blob/main/docs/PROVIDERS.md)
+and its [install instructions](https://github.com/BackendStack21/odek)).
+Later, `bodek upgrade` downloads and installs the latest release the same way.
+
 On Windows, download the `windows_amd64` (or `arm64`) `.zip` from the
-releases page and put `bodek.exe` on your `PATH`.
+[releases page](https://github.com/BackendStack21/bodek/releases) and put
+`bodek.exe` on your `PATH`.
 
 ### From source