Title: fix: write Honcho peer cards with observer target

Summary

• Fixes HonchoSessionManager.set_peer_card() so it writes peer cards through the same observer/target relationship used by get_peer_card().
• Adds a regression test proving set_peer_card(..., peer="user") calls assistant_peer.set_card(card, target=user_peer_id) when AI-observes-user mode is enabled.

Bug

In AI-observes-user mode, peer-card reads use the assistant observer with a user target, e.g.:

assistant_peer.get_card(target=session.user_peer_id)

Before this patch, peer-card writes resolved only the target peer and called:

user_peer.set_card(card)

That made honcho_profile(card=[...], peer="user") report success while a subsequent honcho_profile(peer="user") could still return an empty card, because the read and write paths addressed different Honcho card scopes.

Fix

set_peer_card() now uses _resolve_observer_target() and writes to the observer peer with the resolved target:

observer_peer.set_card(card, target=target_peer_id)

It falls back to bare set_card(card) only when there is no separate target, matching the existing read-side behavior.

Verification

Ran:

python -m pytest tests/honcho_plugin/test_session.py -q -o 'addopts='
python -m pytest tests/honcho_plugin -q -o 'addopts='

Results:

116 passed in 1.97s
267 passed in 2.91s

Local reproduction observed

A one-time Honcho backfill created conclusions successfully, and honcho_search / honcho_reasoning could see them. But honcho_profile(peer="user") returned an empty profile even after honcho_profile(card=[...], peer="user") returned success. After this patch and a fresh Hermes process, honcho_profile(peer="user") returned the seeded facts correctly.