tree: add AGENTS.md router and improve AI instruction files

Add AGENTS.md as a minimal router that directs AI agents to the
relevant instruction files based on what they are editing.

Improve the instruction files:

- cpp.instructions.md: clarify seastarx.hh scope (headers, not
  "many files"), explain std::atomic restriction (single-shard
  model, not "blocking"), scope macros prohibition to new ad-hoc
  only, add coroutine exception propagation pattern, add invariant
  checking section preferring throwing_assert() over SCYLLA_ASSERT
  (issue #7871)
- python.instructions.md: demote PEP 8 to fallback after local
  style, clarify that only wildcard imports are prohibited
- copilot-instructions.md: show configure.py defaults to dev mode,
  add frozen toolchain section, clarify --no-gather-metrics applies
  to test.py, fix Python test paths to use .py extension, add
  license header guidance for new files

Closes scylladb/scylladb#29023

.github/instructions/python.instructions.md

@@ -7,7 +7,7 @@ applyTo: "**/*.py"
 **Important:** Match existing code style. Some directories (like `test/cqlpy` and `test/alternator`) prefer simplicity over type hints and docstrings.
 
 ## Style
-- Follow PEP 8
+- Match style of the file and directory you are editing; fall back to PEP 8 if unclear
 - Use type hints for function signatures (unless directory style omits them)
 - Use f-strings for formatting
 - Line length: 160 characters max
@@ -25,7 +25,7 @@ from cassandra.cluster import Cluster
 from test.utils import setup_keyspace
 ```
 
-Never use `from module import *`
+Avoid wildcard imports (`from module import *`).
 
 ## Documentation
 All public functions/classes need docstrings (unless the current directory conventions omit them):