test(#143): Validate 50% rule prevents context exhaustion

Following TDD (Red-Green-Refactor):
- RED: Created comprehensive test suite with 12 test cases
- GREEN: Implemented validation logic that passes all tests
- All quality gates passed

Test Coverage:
- Oversized issue (120K) correctly rejected
- Properly sized issue (80K) correctly accepted
- Edge case at exactly 50% (100K) correctly accepted
- Sequential issues validated individually
- All agent types tested (opus, sonnet, haiku, glm, minimax)
- Edge cases covered (zero, very small, boundaries)

Implementation:
- src/validation.py: Pure validation function
- tests/test_fifty_percent_rule.py: 12 comprehensive tests
- docs/50-percent-rule-validation.md: Validation report
- 100% test coverage (14/14 statements)
- Type checking: PASS (mypy)
- Linting: PASS (ruff)

The 50% rule ensures no single issue exceeds 50% of target
agent's context limit, preventing context exhaustion while
allowing efficient capacity utilization.

Fixes #143

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/coordinator/src/validation.py

@@ -0,0 +1,74 @@
+"""Issue assignment validation logic.
+
+Validates that issue assignments follow coordinator rules, particularly
+the 50% rule to prevent context exhaustion.
+"""
+
+from dataclasses import dataclass
+
+from .models import IssueMetadata
+
+# Agent context limits (in tokens)
+# Based on COORD-004 agent profiles
+AGENT_CONTEXT_LIMITS = {
+    "opus": 200_000,
+    "sonnet": 200_000,
+    "haiku": 200_000,
+    "glm": 128_000,
+    "minimax": 128_000,
+}
+
+
+@dataclass
+class ValidationResult:
+    """Result of issue assignment validation.
+
+    Attributes:
+        valid: Whether the assignment is valid
+        reason: Human-readable reason if invalid (empty string if valid)
+    """
+
+    valid: bool
+    reason: str = ""
+
+
+def validate_fifty_percent_rule(metadata: IssueMetadata) -> ValidationResult:
+    """Validate that issue doesn't exceed 50% of target agent's context limit.
+
+    The 50% rule prevents context exhaustion by ensuring no single issue
+    consumes more than half of an agent's context window. This leaves room
+    for conversation history, tool use, and prevents hitting hard limits.
+
+    Args:
+        metadata: Issue metadata including estimated context and assigned agent
+
+    Returns:
+        ValidationResult with valid=True if issue passes, or valid=False with reason
+
+    Example:
+        >>> metadata = IssueMetadata(estimated_context=120000, assigned_agent="sonnet")
+        >>> result = validate_fifty_percent_rule(metadata)
+        >>> print(result.valid)
+        False
+    """
+    agent = metadata.assigned_agent
+    estimated = metadata.estimated_context
+
+    # Get agent's context limit
+    context_limit = AGENT_CONTEXT_LIMITS.get(agent, 200_000)
+
+    # Calculate 50% threshold
+    max_allowed = context_limit // 2
+
+    # Validate
+    if estimated > max_allowed:
+        return ValidationResult(
+            valid=False,
+            reason=(
+                f"Issue context estimate ({estimated} tokens) exceeds 50% rule for "
+                f"{agent} agent. Maximum allowed: {max_allowed} tokens "
+                f"(50% of {context_limit} context limit)."
+            ),
+        )
+
+    return ValidationResult(valid=True, reason="")