fix: strip ANSI at the source — clean terminal output before it reaches the model

Root cause: terminal_tool, execute_code, and process_registry returned raw
subprocess output with ANSI escape sequences intact. The model saw these
in tool results and copied them into file writes.

Previous fix (PR #2532) stripped ANSI at the write point in file_tools.py,
but this was a band-aid — regex on file content risks corrupting legitimate
content, and doesn't prevent ANSI from wasting tokens in the model context.

Source-level fix:
- New tools/ansi_strip.py with comprehensive ECMA-48 regex covering CSI
  (incl. private-mode, colon-separated, intermediate bytes), OSC (both
  terminators), DCS/SOS/PM/APC strings, Fp/Fe/Fs/nF escapes, 8-bit C1
- terminal_tool.py: strip output before returning to model
- code_execution_tool.py: strip stdout/stderr before returning
- process_registry.py: strip output in poll/read_log/wait
- file_tools.py: remove _strip_ansi band-aid (no longer needed)

Verified: `ls --color=always` output returned as clean text to model,
file written from that output contains zero ESC bytes.

tests/tools/test_ansi_strip.py

@@ -0,0 +1,168 @@
+"""Comprehensive tests for ANSI escape sequence stripping (ECMA-48).
+
+The strip_ansi function in tools/ansi_strip.py is the source-level fix for
+ANSI codes leaking into the model's context via terminal/execute_code output.
+It must strip ALL terminal escape sequences while preserving legitimate text.
+"""
+
+from tools.ansi_strip import strip_ansi
+
+
+class TestStripAnsiBasicSGR:
+    """Select Graphic Rendition — the most common ANSI sequences."""
+
+    def test_reset(self):
+        assert strip_ansi("\x1b[0m") == ""
+
+    def test_color(self):
+        assert strip_ansi("\x1b[31;1m") == ""
+
+    def test_truecolor_semicolon(self):
+        assert strip_ansi("\x1b[38;2;255;0;0m") == ""
+
+    def test_truecolor_colon_separated(self):
+        """Modern terminals use colon-separated SGR params."""
+        assert strip_ansi("\x1b[38:2:255:0:0m") == ""
+        assert strip_ansi("\x1b[48:2:0:255:0m") == ""
+
+
+class TestStripAnsiCSIPrivateMode:
+    """CSI sequences with ? prefix (DEC private modes)."""
+
+    def test_cursor_show_hide(self):
+        assert strip_ansi("\x1b[?25h") == ""
+        assert strip_ansi("\x1b[?25l") == ""
+
+    def test_alt_screen(self):
+        assert strip_ansi("\x1b[?1049h") == ""
+        assert strip_ansi("\x1b[?1049l") == ""
+
+    def test_bracketed_paste(self):
+        assert strip_ansi("\x1b[?2004h") == ""
+
+
+class TestStripAnsiCSIIntermediate:
+    """CSI sequences with intermediate bytes (space, etc.)."""
+
+    def test_cursor_shape(self):
+        assert strip_ansi("\x1b[0 q") == ""
+        assert strip_ansi("\x1b[2 q") == ""
+        assert strip_ansi("\x1b[6 q") == ""
+
+
+class TestStripAnsiOSC:
+    """Operating System Command sequences."""
+
+    def test_bel_terminator(self):
+        assert strip_ansi("\x1b]0;title\x07") == ""
+
+    def test_st_terminator(self):
+        assert strip_ansi("\x1b]0;title\x1b\\") == ""
+
+    def test_hyperlink_preserves_text(self):
+        assert strip_ansi(
+            "\x1b]8;;https://example.com\x1b\\click\x1b]8;;\x1b\\"
+        ) == "click"
+
+
+class TestStripAnsiDECPrivate:
+    """DEC private / Fp escape sequences."""
+
+    def test_save_restore_cursor(self):
+        assert strip_ansi("\x1b7") == ""
+        assert strip_ansi("\x1b8") == ""
+
+    def test_keypad_modes(self):
+        assert strip_ansi("\x1b=") == ""
+        assert strip_ansi("\x1b>") == ""
+
+
+class TestStripAnsiFe:
+    """Fe (C1 as 7-bit) escape sequences."""
+
+    def test_reverse_index(self):
+        assert strip_ansi("\x1bM") == ""
+
+    def test_reset_terminal(self):
+        assert strip_ansi("\x1bc") == ""
+
+    def test_index_and_newline(self):
+        assert strip_ansi("\x1bD") == ""
+        assert strip_ansi("\x1bE") == ""
+
+
+class TestStripAnsiNF:
+    """nF (character set selection) sequences."""
+
+    def test_charset_selection(self):
+        assert strip_ansi("\x1b(A") == ""
+        assert strip_ansi("\x1b(B") == ""
+        assert strip_ansi("\x1b(0") == ""
+
+
+class TestStripAnsiDCS:
+    """Device Control String sequences."""
+
+    def test_dcs(self):
+        assert strip_ansi("\x1bP+q\x1b\\") == ""
+
+
+class TestStripAnsi8BitC1:
+    """8-bit C1 control characters."""
+
+    def test_8bit_csi(self):
+        assert strip_ansi("\x9b31m") == ""
+        assert strip_ansi("\x9b38;2;255;0;0m") == ""
+
+    def test_8bit_standalone(self):
+        assert strip_ansi("\x9c") == ""
+        assert strip_ansi("\x9d") == ""
+        assert strip_ansi("\x90") == ""
+
+
+class TestStripAnsiRealWorld:
+    """Real-world contamination scenarios from bug reports."""
+
+    def test_colored_shebang(self):
+        """The original reported bug: shebang corrupted by color codes."""
+        assert strip_ansi(
+            "\x1b[32m#!/usr/bin/env python3\x1b[0m\nprint('hello')"
+        ) == "#!/usr/bin/env python3\nprint('hello')"
+
+    def test_stacked_sgr(self):
+        assert strip_ansi(
+            "\x1b[1m\x1b[31m\x1b[42mhello\x1b[0m"
+        ) == "hello"
+
+    def test_ansi_mid_code(self):
+        assert strip_ansi(
+            "def foo(\x1b[33m):\x1b[0m\n    return 42"
+        ) == "def foo():\n    return 42"
+
+
+class TestStripAnsiPassthrough:
+    """Clean content must pass through unmodified."""
+
+    def test_plain_text(self):
+        assert strip_ansi("normal text") == "normal text"
+
+    def test_empty(self):
+        assert strip_ansi("") == ""
+
+    def test_none(self):
+        assert strip_ansi(None) is None
+
+    def test_whitespace_preserved(self):
+        assert strip_ansi("line1\nline2\ttab") == "line1\nline2\ttab"
+
+    def test_unicode_safe(self):
+        assert strip_ansi("emoji 🎉 and ñ café") == "emoji 🎉 and ñ café"
+
+    def test_backslash_in_code(self):
+        code = "path = 'C:\\\\Users\\\\test'"
+        assert strip_ansi(code) == code
+
+    def test_square_brackets_in_code(self):
+        """Array indexing must not be confused with CSI."""
+        code = "arr[0] = arr[31]"
+        assert strip_ansi(code) == code

tests/tools/test_file_tools.py

@@ -309,3 +309,6 @@ class TestSearchHints:
         raw = search_tool(pattern="foo", offset=50, limit=50)
         assert "[Hint:" in raw
         assert "offset=100" in raw
+
+
+