fix(runtime): prevent orphaned tool-result at compaction boundary; /cost JSON

Two fixes:

1. compact.rs: When the compaction boundary falls at the start of a
   tool-result turn, the preceding assistant turn with ToolUse would be
   removed — leaving an orphaned role:tool message with no preceding
   assistant tool_calls. OpenAI-compat backends reject this with 400.

   Fix: after computing raw_keep_from, walk the boundary back until the
   first preserved message is not a ToolResult (or its preceding assistant
   has been included). Regression test added:
   compaction_does_not_split_tool_use_tool_result_pair.

   Source: gaebal-gajae multi-turn tool-call 400 repro 2026-04-09.

2. /cost resume: add JSON output:
   {kind:cost, input_tokens, output_tokens, cache_creation_input_tokens,
    cache_read_input_tokens, total_tokens}

159 CLI + 431 runtime tests pass. Fmt clean.

rust/crates/rusty-claude-cli/src/main.rs

@@ -2744,7 +2744,14 @@ fn run_resume_command(
             Ok(ResumeCommandOutcome {
                 session: session.clone(),
                 message: Some(format_cost_report(usage)),
-                json: None,
+                json: Some(serde_json::json!({
+                    "kind": "cost",
+                    "input_tokens": usage.input_tokens,
+                    "output_tokens": usage.output_tokens,
+                    "cache_creation_input_tokens": usage.cache_creation_input_tokens,
+                    "cache_read_input_tokens": usage.cache_read_input_tokens,
+                    "total_tokens": usage.total_tokens(),
+                })),
             })
         }
         SlashCommand::Config { section } => {