refactor: both tools import the same fragments; scope code comments by path

Codex reads ~/.codex/AGENTS.md, not ~/.agents/AGENTS.md, so the file
generated there last commit was inert. Both tools support @path imports,
which removes the generate-and-drift step entirely.

code-comments.md moves to ~/.claude/rules/ with paths frontmatter, so its
24 lines load only when a source file is read.

claude-md/writing.md

@@ -81,26 +81,6 @@ Commits: Conventional Commits, subject ≤50 chars, body only when the
 "why" is not obvious. Review comments: one line per finding — location,
 problem, fix.
 
-### Code comments
-
-Budget: 1-3 lines. Write one only when an agent with the repo and thirty
-seconds of grep would still get it wrong. That means one of:
-
-- a landmine no test catches
-- a fact not in the repo (deployed bytecode, chain quirk, library bug)
-- a unit or epoch the type cannot carry (wei, ms, 18-dp)
-- a MUST or MUST NOT a caller can break
-
-Good:
-
-> `// MUST read lastGoodPrice before fetchPrice — same eth_call state.`
-
-Everything else: delete. Design rationale and rejected alternatives go in
-`docs/`; leave a path. History is in git. No emphasis; ALL-CAPS only for
-MUST / NEVER.
-
-Deleting a comment is cheap and reversible. When unsure, delete.
-
 ### Where this does not apply
 
 Security warnings, irreversible-action confirmations, and legal or