csharp: the prompt hook never says a C#-only tree has no graph yet, because orient.py's extension list omits .cs #1453

Symptom

In a C#-only tree with no graph, the prompt hook prints nothing. A Java tree of the same shape gets the one-time line saying no call graph exists yet and that axiomcode index builds one. So a C# user who has not indexed never learns the plugin can help, although index builds a C# graph of that tree in about 12 s. Measured on 0.1.3 at 0df257c.

Reproduction

Two directories. Each holds 30 files, one class per file, Widget1 to Widget30, and neither has a .axiomcode/:

cs/Widget1.cs          namespace App;  public class Widget1 { }     (and cs/App.csproj, net8.0)
java/Widget1.java      package app;    public class Widget1 { }

echo '{"cwd":"<dir>","prompt":"what breaks if I change the Widget1 class in this repo","session_id":"t","hook_event_name":"UserPromptSubmit"}' \
  | python3 plugins/axiomcode/hooks/orient.py

• java/: graph: this repository has no call graph yet, so callers, change impact and test selection are unavailable until one is built., followed by the index line
• cs/: no output, exit 0, and no stamp is written

After axiomcode index on a copy of cs/ (csharp 30 files), both axiomcode impact Widget1 and the axiomcode_impact MCP tool answer.

Root cause

plugins/axiomcode/hooks/orient.py:77 counts source files with EXT = ('.java', '.ts', '.tsx', '.py', '.js', '.jsx', '.mjs', '.cjs'). That list does not include .cs, so found stays 0 and line 86 exits silently. Other lists in the plugin do include .cs: SRC in hooks/direct.py:30, PATHISH in orient.py:116, and BY_EXT in skills/axiomcode/scripts/ax_langs.py:35. This is the shape of #1083 and #1076, in a list those fixes did not cover.

Expected

A tree with 25 or more .cs files and no graph gets the same "no call graph yet" line that a Java tree gets.