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.

tools/terminal_tool.py

@@ -1163,6 +1163,11 @@ def terminal_tool(
                 )
                 output = output[:head_chars] + truncated_notice + output[-tail_chars:]
 
+            # Strip ANSI escape sequences so the model never sees terminal
+            # formatting — prevents it from copying escapes into file writes.
+            from tools.ansi_strip import strip_ansi
+            output = strip_ansi(output)
+
             # Redact secrets from command output (catches env/printenv leaking keys)
             from agent.redact import redact_sensitive_text
             output = redact_sensitive_text(output.strip()) if output else ""