fix(pin): guard null content block in isPinReply

[dex0shubham]

Problem

isPinReply (src/core/pin.ts) took the sole content block after a length check and read .type without guarding it:

const blk = blocks[0]!;
return (blk as { type?: string }).type === 'text' && ...

A null content block (e.g. {"role":"assistant","content":[null]} following a @pxpipe pin … command turn — the only path that evaluates isPinReply) passes the length !== 1 check, and blocks[0]!.type throws TypeError: Cannot read properties of null (reading 'type').

The throw is caught at the pin call site, so it doesn't 500 — instead the whole pin feature silently no-ops for that request: the raw @pxpipe pin … command lines stay in the outbound body (forwarded to the model) and the user's pinned rules are not re-emitted at the tail.

Every sibling that walks content blocks in this module guards this first (isCommandOnlyTurn, pinLines, systemPinLines, stripFromMessage all do if (!blk || typeof blk !== 'object')). isPinReply was the exception.

Fix

Add the same !!blk && typeof blk === 'object' guard, so a malformed turn is simply not treated as a pin reply.

Added a regression test (stripPinCommands with a null assistant block after a pin command) — it throws on the old code and passes with the fix.

Full suite green (1207/1207); typecheck clean.