fix: use targeted JSON patching in sync_daemon_session_status

[Wirasm]

Summary

• sync_daemon_session_status() was using save_session_to_file() which round-trips through the Session struct, silently dropping fields from newer binary versions (e.g., task_list_id)
• Same class of bug fixed in agent_status.rs by PR feat: transfer task list across agent sessions via CLAUDE_CODE_TASK_LIST_ID #319, just in a different code path (kild list/kild status daemon sync)
• Replaced full-struct serialization with targeted JSON field patching via new patch_session_json_fields() helper

Changes

File	Change
crates/kild-core/src/sessions/persistence.rs	Add patch_session_json_fields() for atomic multi-field JSON patching
crates/kild-core/src/sessions/handler.rs	Replace save_session_to_file() with field-level patches in sync_daemon_session_status
crates/kild-core/src/sessions/persistence.rs	Add test verifying unknown fields survive multi-field patching

Testing

• cargo fmt --check passes
• cargo clippy --all -- -D warnings passes
• cargo test --all passes (all 1243 tests)
• New test test_patch_session_json_fields_preserves_unknown_fields validates the fix

Validation

cargo fmt --check && cargo clippy --all -- -D warnings && cargo test --all

Fixes #321

[Wirasm]

Self-Review

Summary

Clean, focused fix that correctly addresses the root cause: sync_daemon_session_status() was using save_session_to_file() which round-trips through the Session struct, dropping fields unknown to the current binary version. Replaced with targeted JSON field patching, matching the pattern established in PR #319 for agent_status.rs.

Findings

Strengths

• Root cause correctly identified and fixed (same class of bug as agent_status)
• patch_session_json_fields() mirrors patch_session_json_field() exactly
• Atomic file write preserved (single read + write for multiple fields)
• In-memory session updated unconditionally, matching original behavior
• Comment explains the "why" clearly

Noted (non-blocking)

• patch_session_json_fields() shares the non-object JSON error path with patch_session_json_field() which already has test coverage - no additional test needed per YAGNI
• Other save_session_to_file() call sites (create, open, stop, complete) are safe - they write the full Session on initial creation where no unknown fields can exist

Security

• No security concerns

Checklist

• Fix addresses root cause from investigation
• Code follows codebase patterns (mirrors PR feat: transfer task list across agent sessions via CLAUDE_CODE_TASK_LIST_ID #319 fix)
• Test covers the change (unknown field preservation)
• No bugs introduced
• All 1243 tests pass, clippy + fmt clean