Artifacts get 'path pins' to catch stale writes, plus an overwrite_unread escape hatch

What

• A new "path pins" system tracks the sha256 hash and version of every artifact file Claude has read or listed. Publishing (writing or removing) a file is now refused if the live file has changed since Claude last saw it, or if Claude never read it at all, with explicit "Not published"-style messages prompting Claude to re-read before retrying. Batched file reads are capped, with an exemption for live files.
• This state (pathPins, pathPinAck, frozenPathPin, pathPinSeq) is cleared on session reset, unless the reset continues the same conversation.
• A new overwrite_unread parameter on artifact publish (for both files and root publishes) lets a caller explicitly name files to replace or remove even though Claude hasn't read or listed them this session, for cases where the user explicitly asked for an outright replace.

Why This prevents Claude from accidentally clobbering artifact changes it hasn't seen, for example edits made by someone else since Claude last looked, while still allowing an explicit, deliberate overwrite when that is genuinely what the user wants.