[Coding Agent] Add find-allocation-opportunities skill

[lucaspimentel]

Summary of changes

Add a Claude Code skill that scans code for heap allocation opportunities in hot paths, codifying 22+ proven allocation-avoidance patterns used by the team.

Reason for change

The dd-trace-dotnet tracer runs in-process with customer applications, so heap allocations in hot paths directly impact customer GC pressure. The team has accumulated significant tribal knowledge about allocation-avoidance patterns. This skill makes that knowledge consistently available to AI-assisted code reviews and allocation audits.

Implementation details

• SKILL.md: Skill definition with triggering keywords, assessment criteria (severity levels, confidence thresholds), and structured output format for allocation findings
• references/patterns.md: 22+ proven allocation-avoidance patterns with before/after examples (e.g., ArraySegment<T> over Array.Copy, struct enumerators, TryFormat over ToString(), tag list pre-sizing)
• references/anti-patterns.md: Common allocation anti-patterns organized by category (string operations, collections, LINQ, boxing, closures, async, logging) with severity ratings and fixes

Test coverage

N/A — skill definition files only (no compiled code).

Other details

The skill covers both scanning existing code and reviewing PR diffs for missed optimization opportunities. It prioritizes findings by hot-path proximity and uses confidence-based filtering to minimize false positives.