Bump version to 0.7.3

[edwin-zvs]

Summary

• Bump workspace package version to 0.7.3
• Update Cargo.lock package versions for the release

Tests

• cargo build
• cargo test -p agentd move_session_ignores_hidden_subagents_in_reorder_region

Release follow-up after merge: tag the merge commit as v0.7.3 and push the tag.