fix(mcp): reversible text truncation; bound expand_query floor

Address truncation-audit findings so every markdown-default response stays
bounded and recoverable:

- S4: tracedecay_files and tracedecay_type_hierarchy no longer clip oversized
  output irreversibly. New render::truncate_text_with_handle stores the full
  body and returns the markdown truncation envelope with an rh_ handle
  (delegates to truncated_markdown_with_handle; no duplicated logic).
- S3: lcm_expand_query_tool_json's format:"json"+needs_synthesis path could
  emit an unbounded payload when even the Minimal compaction tier overflowed.
  New bounded_lcm_expand_query_floor_text enforces a floor: stores a retrieval
  handle (surfaced as response_handle for the Hermes bridge) and keeps the
  contract keys (status, needs_synthesis, bounded answer/prompt, rebuilt
  synthesis_prompt) while zeroing unbounded arrays with *_truncated markers.
- S5: LCM preflight/expand_query compaction tiers now call
  observe_response_truncation so those truncations show up in metrics.
- Drop unused LCM_GREP_SNIPPET_CHARS (lcm_grep hits are already bounded).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ScriptedAlchemy

src/mcp/tools/handlers/info.rs

@@ -13,7 +13,7 @@ use crate::storage::{ProjectPath, StorageMode, StoreKind};
 use crate::tracedecay::{BranchDiagnostics, TraceDecay};
 use crate::types::{NodeKind, Visibility};
 
-use super::super::render::{self, truncate_response, Md};
+use super::super::render::{self, Md};
 use super::super::ToolResult;
 use super::support::{effective_path, filter_by_scope, require_node_id, unique_file_paths};
 
@@ -632,7 +632,7 @@ pub(super) async fn handle_files(
 
     Ok(ToolResult::new(
         json!({
-            "content": [{ "type": "text", "text": truncate_response(&output) }]
+            "content": [{ "type": "text", "text": render::truncate_text_with_handle(Some(cg.project_root()), &output) }]
         }),
         touched_files,
     ))
@@ -1535,7 +1535,7 @@ pub(super) async fn handle_type_hierarchy(cg: &TraceDecay, args: Value) -> Resul
 
     let touched_files = unique_file_paths(all_files.iter().map(std::string::String::as_str));
     Ok(ToolResult::new(
-        json!({"content": [{"type": "text", "text": truncate_response(&output)}]}),
+        json!({"content": [{"type": "text", "text": render::truncate_text_with_handle(Some(cg.project_root()), &output)}]}),
         touched_files,
     ))
 }