fix(learn): keep traceback tail in tool-error digest preview (#2596)

## Description

`_format_tool_call` in `headroom/learn/analyzer.py` built the error
preview with a head-only slice — `tc.output[:200]`. For tracebacks the
root cause (`ExceptionType: message`) is at the **tail**, so the digest
showed only `Traceback (most recent call last):` plus the first frame
and dropped the actual diagnosis. The issue reports 46% of 715 measured
errors were truncated past the 200-char head.

Closes #2590

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)

## Changes Made

- Added `_truncate_head_tail()` helper that collapses newlines and, when
over budget, keeps both the head and the tail joined by `…`.
- `_format_tool_call` now uses it for error output so the exception line
survives truncation. Short errors are returned unchanged (no marker).

## Testing

- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check .`)
- [ ] Type checking passes (`mypy headroom`)
- [x] New tests added for new functionality

### Test Output

```text
$ uv run pytest tests/test_learn/test_analyzer.py::TestDigestBuilder -q
9 passed in 1.47s

$ uv run ruff check headroom/learn/analyzer.py tests/test_learn/test_analyzer.py
All checks passed!
```

## Real Behavior Proof

- Environment: headroom @ main, Python 3.14, uv
- Exact command / steps: added a long synthetic traceback (`KeyError:
'the-actual-root-cause'` at the tail) as a failing tool call and built
the digest.
- Observed result: digest now contains both `Traceback` and `KeyError:
'the-actual-root-cause'`, separated by `…`; short errors have no `…`.
- Not tested: mypy not run locally.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective
- [x] New and existing unit tests pass locally with my changes
- [x] I did **not** edit `CHANGELOG.md`

## Additional Notes

Truncation budget stays at 200 chars (now split head/tail). mypy not run
locally; happy to adjust if CI flags anything.

headroom/learn/analyzer.py

@@ -371,6 +371,26 @@ def _format_event(event: SessionEvent) -> str | None:
     return None
 
 
+_ERROR_PREVIEW_MAX = 200
+
+
+def _truncate_head_tail(text: str, max_chars: int = _ERROR_PREVIEW_MAX) -> str:
+    """Collapse newlines and truncate, keeping both the head and the tail.
+
+    A head-only slice drops the end of a traceback, which is exactly where the
+    root cause (``ExceptionType: message``) lives, so the digest would show only
+    the preamble and lose the diagnosis (see #2590). Keep both ends instead.
+    """
+    text = text.replace("\n", " ").strip()
+    if len(text) <= max_chars:
+        return text
+    sep = " … "
+    keep = max_chars - len(sep)
+    head = keep // 2
+    tail = keep - head
+    return f"{text[:head].rstrip()}{sep}{text[-tail:].lstrip()}"
+
+
 def _format_tool_call(tc: ToolCall) -> str:
     """Format a single tool call into a compact digest line."""
     status = "ERROR" if tc.is_error else "OK"
@@ -380,8 +400,9 @@ def _format_tool_call(tc: ToolCall) -> str:
     input_str = tc.input_summary[:120]
 
     if tc.is_error:
-        # Include truncated error output for failures
-        output_preview = tc.output[:200].replace("\n", " ").strip()
+        # Include truncated error output for failures, keeping the tail so a
+        # traceback's root cause survives (#2590).
+        output_preview = _truncate_head_tail(tc.output)
         return f"  [{tc.msg_index}] {tc.name}: {input_str} → {status}{error_cat}: {output_preview}"
     else:
         # Just indicate success with size

tests/test_learn/test_analyzer.py

@@ -155,6 +155,54 @@ class TestDigestBuilder:
         digest = _build_digest(_project(), [])
         assert "0 sessions" in digest or "test-project" in digest
 
+    def test_long_error_output_preserves_tail_root_cause(self):
+        # A traceback's diagnosis lives at the tail; a head-only slice would drop it (#2590).
+        traceback = (
+            "Traceback (most recent call last):\n"
+            + "\n".join(
+                f'  File "mod{i}.py", line {i}, in fn{i}\n    call_{i}()' for i in range(40)
+            )
+            + "\nKeyError: 'the-actual-root-cause'"
+        )
+        sessions = [
+            SessionData(
+                session_id="s1",
+                tool_calls=[
+                    _tc(
+                        name="Bash",
+                        output=traceback,
+                        is_error=True,
+                        error_category=ErrorCategory.UNKNOWN,
+                        msg_index=0,
+                    )
+                ],
+            )
+        ]
+        digest = _build_digest(_project(), sessions)
+        assert "Traceback" in digest
+        assert "KeyError: 'the-actual-root-cause'" in digest
+        assert "…" in digest
+
+    def test_short_error_output_not_truncated(self):
+        short = "ModuleNotFoundError: No module named 'foo'"
+        sessions = [
+            SessionData(
+                session_id="s1",
+                tool_calls=[
+                    _tc(
+                        name="Bash",
+                        output=short,
+                        is_error=True,
+                        error_category=ErrorCategory.MODULE_NOT_FOUND,
+                        msg_index=0,
+                    )
+                ],
+            )
+        ]
+        digest = _build_digest(_project(), sessions)
+        assert short in digest
+        assert "…" not in digest
+
 
 # =============================================================================
 # Prior Patterns Injection Tests