docs(readme): fix cold-start quick-start sequence — set API key before prompt, add claw doctor step

The previous quick start jumped from 'cargo build' to 'claw prompt' without
showing the required auth step or the health-check command. A user following
it linearly would fail because the prompt needs an API key.

Changes:
- Numbered steps: build -> set ANTHROPIC_API_KEY -> claw doctor -> prompt
- Windows note updated to show cargo run form as alternative
- Added explicit NOTE that Claude subscription login is not supported (pre-empts #claw-code FAQ)

Source: cold-start friction observed from mezz/mukduk and kapcomunica in #claw-code 2026-04-09.

Author: code-yeongyu

README.md

@@ -49,25 +49,27 @@ The canonical implementation lives in [`rust/`](./rust), and the current source
 > **`cargo install clawcode` will not work** — this package is not published on crates.io. Build from source as shown below.
 
 ```bash
+# 1. Clone and build
 git clone https://github.com/ultraworkers/claw-code
 cd claw-code/rust
 cargo build --workspace
-./target/debug/claw --help
-./target/debug/claw prompt "summarize this repository"
-```
 
-> [!NOTE]
-> **Windows (PowerShell):** the binary is `claw.exe`, not `claw`. Use `.\target\debug\claw.exe` or just run `cargo run -- --help` to skip the path lookup.
+# 2. Set your API key (Anthropic API key — not a Claude subscription)
+export ANTHROPIC_API_KEY="sk-ant-..."
 
-Authenticate with either an API key or the built-in OAuth flow:
+# 3. Verify everything is wired correctly
+./target/debug/claw doctor
 
-```bash
-export ANTHROPIC_API_KEY="sk-ant-..."
-# or
-cd rust
-./target/debug/claw login
+# 4. Run a prompt
+./target/debug/claw prompt "say hello"
 ```
 
+> [!NOTE]
+> **Windows (PowerShell):** the binary is `claw.exe`, not `claw`. Use `.\target\debug\claw.exe` or run `cargo run -- prompt "say hello"` to skip the path lookup.
+
+> [!NOTE]
+> **Auth:** claw requires an **API key** (`ANTHROPIC_API_KEY`, `OPENAI_API_KEY`, etc.) — Claude subscription login is not a supported auth path.
+
 Run the workspace test suite:
 
 ```bash