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
Closesscylladb/scylladb#29023
