refactor: extract feature reducers

[zatevakhin]

Scope

• Extract bounded reducers, actions, and outcomes for mesh/auth, models/profiles, sessions, delegation, and chat stream/tool/usage/error/elicitation/history behavior.
• Make ordered coordination and effects explicit while keeping application composition shallow.
• Include the release coordination fix and the SessionNotification -> AppEvent -> reducer acceptance bridge.
• Exclude Phase 8+ work.

Validation

• cargo fmt --all -- --check
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --all-targets --all-features
• Debug all-target/all-feature tests: 999 passed, 0 failed/ignored.
• Release all-target/all-feature tests: 999 passed, 0 failed/ignored.
• Exact approved head: d97bf1b2d5c4bed656431bb039d6ca6a2c52e6cd.
• Independent aggregate review approved with no actionable findings.

Residual

• No live ACP/UI smoke was available.

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