Centralize g3 status message formatting

Extract a new g3_status module in g3-cli that provides consistent formatting
for all 'g3:' prefixed system status messages.

Key changes:
- Add G3Status struct with methods for progress, done, failed, error, etc.
- Add Status enum with Done, Failed, Error, Resolved, Insufficient, NoChanges
- Add ThinResult struct in g3-core for semantic thinning data
- Update UiWriter trait with print_thin_result() method
- Refactor context thinning to return ThinResult instead of formatted strings
- Update all callers to use the new centralized formatting
- Session resume/decline messages now use G3Status
- Compaction status messages now use G3Status

This maintains clean separation of concerns: g3-core emits semantic data,
g3-cli handles all terminal formatting and colors.

crates/g3-core/src/compaction.rs

@@ -234,9 +234,9 @@ fn apply_summary_fallback_sequence<W: UiWriter>(
     
     // Step 1: Try thinnify (first third of context)
     ui_writer.print_context_status("🥒 Step 1: Trying thinnify...\n");
-    let (thin_msg, chars_saved) = context_window.thin_context(None);
-    thinning_events.push(chars_saved);
-    ui_writer.print_context_thinning(&thin_msg);
+    let thin_result = context_window.thin_context(None);
+    thinning_events.push(thin_result.chars_saved);
+    ui_writer.print_thin_result(&thin_result);
     
     // Recalculate after thinnify
     let (new_max, still_needs_reduction) = provider_config::calculate_summary_max_tokens(
@@ -253,9 +253,9 @@ fn apply_summary_fallback_sequence<W: UiWriter>(
     
     // Step 2: Try skinnify (entire context)
     ui_writer.print_context_status("🦴 Step 2: Trying skinnify...\n");
-    let (skinny_msg, chars_saved) = context_window.thin_context_all(None);
-    thinning_events.push(chars_saved);
-    ui_writer.print_context_thinning(&skinny_msg);
+    let skinny_result = context_window.thin_context_all(None);
+    thinning_events.push(skinny_result.chars_saved);
+    ui_writer.print_thin_result(&skinny_result);
     
     // Recalculate after skinnify
     let (final_max, final_needs_reduction) = provider_config::calculate_summary_max_tokens(