Artifact writes gain optional version-conflict checking

What

• Artifact database writes (both single writes and batch writes) can now pass a requireVersion flag to the server, asking it to reject a write if the artifact has changed since the client last read it.
• Writes made through the str_replace editing path also pass requireVersion; this only happens when the artifact-DB feature (CLAUDE_CODE_ARTIFACT_DB, on by default) is enabled and either the CLAUDE_CODE_ARTIFACT_DB_STR_REPLACE environment variable or a remote flag (tengu_umber_stile, off by default) is set.
• If a server does not understand requireVersion, the client remembers that server as unsupported and automatically retries the write without requireVersion instead of failing.

Why This adds an opt-in safeguard against silently overwriting someone else's concurrent changes to an artifact, while staying compatible with older servers that don't support the check yet.