v0.5.1 — fix group misattribution (user: ownership tag)

Fixes worsening group-chat misattribution (张冠李戴 / 认错人·串记忆).

Root cause (confirmed from a live reasoning trace): the persona's identity-anchoring rule (§11) only accepts injected memory entries tagged user: <id> matching the current speaker; the ReadingSteiner_Memory block carried no user: tag, so the model fell back to guessing ownership — and as §11 grew stricter, the mismatch surfaced more often.

Fix: the plugin only ever injects the current user's own memory (search is scoped to user_id=ident.user_id), so the whole block belongs to the current speaker. It now emits a user: <id> ownership header as the block's first line, which §11 adopts with confidence. Gated by inject_user_attribution (default on); applies to both auto-injection and the epk_recall tool path.

See CHANGELOG.md.