fix: restore OpenCode user messages after app restart

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Bug and impact

OpenCode sessions lost all user messages (initial prompt and follow-ups) after restarting Orbit or reopening a session. Assistant replies and tool output were still restored from the database, but the chat feed showed only agent-side content — a significant data-loss regression for OpenCode users.

Trigger scenario:

1. Create an OpenCode session and send one or more prompts
2. Restart Orbit (or reload the session from DB)
3. Open the session — user messages are missing from the chat

Root cause

Commit 7cc5ce2 added "user" => {} in process_line_opencode to prevent duplicate first messages when OpenCode echoes user JSONL on stdout during live sessions. That blanket ignore also applied when replaying session_outputs from SQLite on restore, where user lines are the only source of user history.

Claude/Codex sessions were unaffected because they use different line processors that still handle user JSONL.

Fix

• Added replay_opencode_user_line() to parse Orbit-persisted user JSONL (message.content as string) during journal restore
• Skip only immediate duplicates (last journal entry is the same user text) to suppress live OpenCode echoes without dropping distinct follow-ups
• Added unit/integration tests for restore, echo dedup, and follow-up after assistant reply
• Minor clippy fix in process_util.rs (unused cmd on non-Windows)

Validation

• cargo test --manifest-path tauri/Cargo.toml --lib — 183 passed
• cargo clippy --manifest-path tauri/Cargo.toml -- -D warnings — clean