refactor(e2e-ui): session guards on user command mounts

[patrickleet]

Summary

• Wire e2e-ui todo/blob/chat commands with .guarded(causal_has_user, …) (and causal_is_admin for force-archive) instead of discovering missing identity mid-handler.
• Handlers use principal(ctx) for domain owner/author args only; input parse, load, and domain invariants stay in the body.
• Expose CausalCommandContext::session(); treat empty user id as missing for user_id().
• Clarify blob move docs (provisional client pure vs Atomic seal); suite unauth case accepts guard rejection (400/errors).

Stack

Stacked on tasks--contract-lifecycle-tooling-1 (PR #172).

Test plan

• cargo check -p e2e-service
• cargo test -p e2e-service --lib
• session/guard unit tests
• CI offline suite on PR

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