test: add contract tests for CLI output parsing

Implement contract testing to catch CLI schema drift early:

Contract Test Suite (fixture_contract_tests.rs):
- parse_lore_me_empty_fixture: Validates LoreMeResponse on empty data
- parse_lore_me_with_activity_fixture: Real lore output with activity
- parse_br_list_empty_fixture: Empty beads list
- parse_br_list_with_beads_fixture: Real br output with beads

Fixtures (captured from real CLI output):
- fixtures/lore/me_empty.json: Synthetic empty response
- fixtures/lore/me_with_activity.json: Real 'lore --robot me' output
- fixtures/br/list_empty.json: Empty array []
- fixtures/br/list_with_beads.json: Real 'br list --json' output
- fixtures/br/bv_triage.json: Real 'bv --robot-triage' output

Fixture Regeneration:
- scripts/regenerate-fixtures.sh: Captures fresh CLI output
  - Run periodically to update fixtures
  - CI can diff against committed fixtures to detect drift

Why Contract Tests Matter:
MC depends on external CLIs (lore, br, bv) whose output format may
change. Contract tests fail fast when our Rust types diverge from
actual CLI output, preventing runtime deserialization errors.

The tests use include_str!() for compile-time fixture embedding,
ensuring tests fail to compile if fixtures are missing.

src-tauri/tests/fixtures/br/list_empty.json

@@ -0,0 +1 @@
+[]

src-tauri/tests/fixtures/lore/me_empty.json

@@ -0,0 +1,10 @@
+{
+  "ok": true,
+  "data": {
+    "open_issues": [],
+    "open_mrs_authored": [],
+    "reviewing_mrs": [],
+    "activity": [],
+    "since_last_check": null
+  }
+}