fix(acp): abort orphaned agent loops and deliver /review output

[bug-ops]

Summary

• do_close_session/do_delete_session previously only fired cancel_signal.notify_one() without joining or aborting the session's agent-loop task, so a session/load/session/resume reusing the same SessionId could race a still-running old loop and corrupt the new turn's event stream. SessionEntry now tracks the loop's JoinHandle; close/delete abort and await it (5s bounded timeout) before the entry is dropped, and Drop for SessionEntry also aborts it unconditionally as a safety net covering the LRU-eviction/reaper removal paths.
• /review previously dispatched fire-and-forget via input_tx.try_send and returned EndTurn immediately, bypassing acquire_prompt_channels. After PR fix(acp): close PromptChannelGuard reload race and stale-event leak #6672's inter-turn drain fix, /review's output was silently discarded instead of leaking into the next turn. /review is now intercepted in do_prompt and routed through the normal acquire_prompt_channels/drain turn path like any other prompt, so its output actually reaches the client. This is a client-visible behavior change: /review now participates in the same turn-contention check as any other prompt (can be rejected with "prompt already in progress") and its expanded prompt is persisted/replayed as part of session history.

Both issues were follow-up findings from PR #6672's review. Went through two rounds of adversarial critique: the first found a real gap where set_agent_loop_handle could silently drop a JoinHandle if the session entry was removed before the loop was fully wired up (fixed: aborts immediately instead), and a /review trailing-whitespace parsing regression (fixed: trim both ends to match handle_slash_command's existing parity). The second round re-verified both fixes and found no new gaps.

Closes #6674
Closes #6673

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" (zeph-acp, zeph-common, zeph-core scopes) — 2736 passed
• cargo nextest run full zeph-acp package scope (including tests/integration.rs, not just --lib --bins) — 235/235 passed, including all 5 new/changed regression tests:
  • close_session_aborts_agent_loop_task, delete_session_aborts_agent_loop_task (prove the old loop is actually aborted+joined, not just signaled)
  • review_command_output_is_delivered_to_client (positive round-trip proving /review's output reaches the client)
  • review_command_rejects_when_a_turn_is_already_in_progress (loops over ["/review", "/review\n", "/review\t"] to lock in the trim fix)
  • review_command_threads_connection_owner_key_into_channel_message (now also asserts the forwarded text matches the expanded review prompt)
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• gitleaks protect --staged
• CHANGELOG.md updated under [Unreleased]/Fixed