docs: add usage-phases.md beside usage-simple.md in python/configs/

- Rename python/configs/usage.md → usage-simple.md so both scripts have a
  parallel runnable reference file in the same directory
- Add python/configs/usage-phases.md: runnable sh commands + real expected
  output for phases.py against every error-path config, including a note on
  the key difference from simple.py (HTTP 4xx exits 0 in phases.py vs 1 in
  simple.py because phases.py completes at the network level)
- Update doc cross-links in docs/usage/py-simple.md and py-phases.md

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/usage/py-phases.md

@@ -124,18 +124,10 @@ Multiple URLs are separated by a blank line, matching Go's text output style.
 ## Using the error-path example configs
 
 The `python/configs/` files from `simple.py` work identically with `phases.py`.
-Compare the error handling between the two tools:
-
-```sh
-# DNS failure
-python phases.py configs/dns-failure.txt
-
-# Connection refused — note DNS phase is present, TCP is partial
-python phases.py configs/connection-refused.txt
-
-# TLS errors — all three phases up to TLS are present
-python phases.py configs/tls-errors.txt
-```
+See [`python/configs/usage-phases.md`](../../python/configs/usage-phases.md) for
+runnable shell commands and expected output for every config file, including a
+side-by-side comparison of how `phases.py` and `simple.py` differ on HTTP 4xx
+responses.
 
 ## Limitations