fix(herdr): use targeted splits for mirrored reviews

[Shujakuinkuraudo]

What changed

herdr open and herdr last use a targeted split when invoked through Herdr Mirror and the configured placement is the default overlay. This puts the review beside the invoking remote pane even when the remote server is focused on another workspace. Explicit command-line/environment placement overrides retain their precedence, and native local actions retain their overlay default.

An empty focused_pane_agent is also treated as no agent. Mirror uses an empty string to prevent Herdr 0.8.2 from backfilling an unrelated globally focused agent when the invoking pane is a shell.

Fixes #62. The companion local-focus/rendering work is tracked in nikok6/herdr-mirror#98.

Validation

cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace, and python3 herdr/test-manifest.py pass. The Herdr tests cover document/reply placement, exact transcript preservation, explicit placement overrides, native overlay behavior, and a shell context with an empty agent label.

Isolated and real-SSH Herdr 0.8.2 / Mirror 0.4.2 / Annotate 0.3.0 smokes also exercised the same placement behavior through a packaging adapter around the official Plannotator TUI 0.6.0 binary: actual prefix shortcuts reached the invoking remote tab, and document-tree input switched files. Those smokes include the separate Mirror fixes; this PR alone addresses placement, not local Mirror focus.

[backnotprop]

Merged. I confirmed the Herdr side live before merging: Herdr refuses a target for overlay panes and always opens them in the active tab, while a targeted split lands beside the intended pane, so the split default under Mirror is the right call. Thanks for another precise fix.