vendors: add a zed adapter

[jothimani-rajendran]

agentseam carries a zed row in its capability matrix. chock supports 15 agents and
zed is not among them — so a policy authored here cannot be compiled for Zed at all, even
though we already know what Zed's surface looks like.

That makes this an unusually well-scoped adapter task: the research is already done upstream.

What to do:

1. Read zed's row in agentseam's matrix (src/agentseam/matrix.py) for what its hooks can
   really do. Note its pre_tool grade before you plan the work — it is none, which means
   the honest surfaces here are ambient-rule, git-hook and ci-gate, not pre-tool-use.
2. Add the vendor under src/chock/vendors/, following an existing adapter with the same
   surface set. aider, replit and junie are all three-surface adapters and are the closest
   models.
3. Credit only the surfaces Zed actually reaches, in SURFACE_AGENTS.

The rule that governs this: never credit a surface the agent cannot really receive. chock's
whole pitch is that it does not overclaim, and the pinning tests exist to catch exactly that —
chock check --only matrix will tell you if the claim and the code disagree.

Related: agentseam has an open issue to witness Zed on a real run rather than trusting vendor
docs. If that lands first and contradicts the none grade, this adapter gets a fourth surface —
so it is worth checking that issue's state before you start.

Sized to an afternoon. Files: src/chock/vendors/, src/chock/compile/surfaces.py, and the
pinning tests that assert the supported-agent list.