fix: close malformed frontmatter gaps

[zeke-ricon]

Fix-it for KnickKnackLabs#52 review findings:

• make malformed id: values fail closed instead of silently falling back to the message index;
• make malformed ts: values return the generic chat-format error instead of a Python traceback;
• update the generated README's format/cursor examples from the old ###/line-number model to frontmatter/message-index language;
• add regression tests for the malformed id: and ts: cases.

Validation:

• bats test/chat_lib.bats --filter 'message_count: (non-integer id|invalid timestamp|invalid payload)'
• mise run test (196/196)
• mise run readme --check
• python3 -m py_compile lib/parse.py lib/chat_query.py lib/read_query.py
• git diff --check
• codebase lint "$PWD"