v0.5.0 — sub-session learning gate

• Sub-session learning gate: requests carrying a parent-session marker (X-Infinitum-Parent-Session-ID or the bare x-parent-session-id alias) no longer enqueue learning jobs by default, so delegated sub-agent scaffolding stops diluting the memory store. Controlled by learning.skip_subsessions (default true); events and retrieval are unaffected.
• Header precedence for the gate: explicit X-Infinitum-Learning: off beats explicit on, which beats the sub-session skip, which beats default learning; marker headers are stripped before upstream forwarding like the other runtime control headers.
• Docker documentation now defaults to the latest image tag and documents how to pin a specific release tag.