fix(compat): guard .meta.json parser against missing optional fields from older sessions

[delexw]

Summary

Fixes #107 — session-meta sidecar files (.meta.json) written by older Claude Code builds were missing the optional fields that newer builds add. Any strict typed parser would error on these older files.

Changes

src-tauri/src/parser/subagent.rs

• Introduces SidecarMeta — a typed serde struct for .meta.json sidecar files. Every field carries #[serde(default)] or is Option<T>, so files written by older Claude Code builds (which omit new fields) continue to parse without error.
• Replaces the ad-hoc serde_json::Value navigation in read_agent_type with a clean deserialisation into SidecarMeta.
• Adds a doc-comment on SidecarMeta encoding the backwards-compatibility rule: new fields from meta files must always be optional to preserve backwards compatibility with sessions written by older Claude Code builds.

Regression tests (6 new)

Test	What it verifies
sidecar_meta_minimal_json_parses_without_error	Minimal file (only agentType) parses without error
sidecar_meta_empty_object_parses_without_error	Empty {} does not panic
sidecar_meta_unknown_new_fields_are_ignored	Future unknown fields are silently ignored
read_agent_type_returns_empty_for_missing_meta_file	Missing file returns empty string gracefully
read_agent_type_returns_empty_for_empty_object	Empty object returns empty string
read_agent_type_parses_known_field	Known agentType field is read correctly
read_agent_type_tolerates_extra_fields_from_new_claude_code_version	v2.1.149+ extra fields do not break parsing

Test results

• Rust: 404 passed, 0 failed
• Frontend: 349 passed, 0 failed
• Clippy: 0 warnings
• TypeScript: 0 errors