Agent-box log directory is a shared /tmp path: decide the per-user layout #3682

Question (owner decision needed)

`internal/agentbox/process.go` (`DefaultLogDir = "/tmp/agent-box"`) and `internal/coderun/ops.go` (`RunLogDir = "/tmp/agent-box"`) write run logs to one shared, predictable directory in `/tmp`. Clients may read that path, so a different tenant can see another tenant's log, and a planted symlink there is followed.

Options:

• A. Keep the path, add a per-user subdirectory (created 0700 and owned by that user).
• B. Move to a per-run directory from `mktemp -d`, and return the path to the caller.
• C. Keep the path but make the directory root-owned, 0755, and mark clients as read-only. Smallest; keeps client reads of the shared path.

Default if no answer: A.

Acceptance criteria (after the decision)

• Logs for one user are not readable by another user.
• Test: the directory is created with the chosen mode and owner; a symlink planted at the path is refused.

Parked on #3634; follow-up to #3342.

Blocked by: none
Sprint: #3725
Part of #3725