fix: deduct tier header tokens from budget in _build_package

The '### Relevant files' and '### Depends on' headers plus their
trailing blank lines were appended without subtracting their token
cost from remaining. Three unaccounted headers compound to ~30-40
tokens of budget overrun. Now each tier deducts header_cost before
iterating entries.

Skipped singleton accessor nitpick: dependency_analyzer.py,
style_analyzer.py, and dna_extractor.py are all plain classes
instantiated in dependencies.py -- no get_*() accessor pattern
exists in the project. ContextAssembler already matches.

Author: DevanshuNEU

backend/services/context_assembler.py

@@ -237,7 +237,10 @@ def _build_package(
 
         # Tier 1: Relevant files (highest priority)
         if found_files and remaining > 50:
-            tier_lines = ["### Relevant files"]
+            header = "### Relevant files"
+            header_cost = _estimate_tokens(header) + 1  # +1 for trailing blank line
+            remaining -= header_cost
+            tier_lines = [header]
             for r in search_results:
                 fp = r.get("file_path", "")
                 name = r.get("qualified_name", r.get("name", ""))
@@ -257,7 +260,10 @@ def _build_package(
 
         # Tier 2: Dependency files
         if dep_files and remaining > 50:
-            tier_lines = ["### Depends on"]
+            header = "### Depends on"
+            header_cost = _estimate_tokens(header) + 1
+            remaining -= header_cost
+            tier_lines = [header]
             for fp in dep_files[:10]:
                 entry = f"- `{fp}`"
                 entry_tokens = _estimate_tokens(entry)