docs: record list-of-str sysctls/volumes refusals as a decision

[lesnik512]

Re-measured the two remaining list-of-str conformance over-rejections (sysctls: ["a"], volumes: ["a"]) against docker compose config v5.1.2 and podman 6.0.1. Both are legitimate refusals under rule two — podman genuinely cannot form the flag — not unfinished parsers:

• sysctls: ["a"] → Docker {a: ""}; podman rejects --sysctl a= (sysctl 'a' is not allowed) and --sysctl a (must be in the form of KEY=VALUE).
• volumes: ["a"] → Docker anonymous volume with relative target a; podman rejects -v a / --mount ...target=a (must be an absolute path). compose2pod's error already mirrors podman's.

A "won't-fix because podman can't express it" is a settled rejected option, not unscheduled work — so it belongs in decisions/, not deferred.md. This adds decisions/2026-07-16-list-of-str-refusals.md (listed by just index), removes the item from deferred.md's "unfinished parsers" section, and leaves a one-line pointer there so the harness's over-reject reader stays oriented. Docs-only.

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