Add --json output mode to send

[BigCatMellow]

What

Adds an opt-in --json flag to hcom send that prints a single line of JSON on success instead of the normal human-readable feedback:

{"event_id": 3, "delivered_to": ["nova"]}

Why

send_message() already computes the event ID it just logged (db.log_event(...)), but the value is discarded (let _event_id = ...) and the function's return type only ever exposed delivered_to. There's currently no way for a script or another tool wrapping hcom send to learn which exact database event a given send became — a caller would have to guess (e.g. "read the newest event afterward"), which isn't safe under concurrency or retries.

How

• send_message()'s return type changes from Result<Vec<String>, String> to Result<(i64, Vec<String>), String> so the already-computed event ID survives the call. All call sites (the one production call in cmd_send, plus the unit tests in this file that call send_message() directly) are updated to match.
• cmd_send() gets a new if args.json { ...; return 0; } branch inserted immediately before the existing if args.quiet { ... } check — structurally parallel to how --quiet already provides an alternate output mode. When --json is not passed, execution reaches the exact same code as before this change.
• Error paths are unaffected: --json doesn't change anything on a failed send (still Error: ... to stderr, exit 1).

Testing

• cargo build — clean, no warnings.
• cargo clippy --all-targets -- -D warnings — clean.
• cargo test — 2169 passed, 3 pre-existing failures unrelated to this change (confirmed via git stash + rerun against the unpatched tree: db::tests::test_open_raw_rejects_non_temp_path, db::tests::test_open_raw_rejects_temp_symlink_to_non_temp_path, paths::tests::test_is_test_temp_path_rejects_non_temp — all fail identically without this diff, caused by running the checkout under a temp path).
• Manually verified end-to-end: sequential send --json calls print monotonic real event IDs, --json output is exactly one parseable JSON line with nothing else on stdout, --json --quiet together still prints exactly one JSON line, and error paths are unaffected.

Kept the diff as small as possible — one file, no unrelated changes.