atomicWriteFile skill installation

Chose same-directory temporary writes plus rename() over direct skill-file writes because POSIX readers must never observe truncation. Install only to ~/.agents/skills/sentry-cli/ and ~/.claude/skills/sentry-cli/; OpenCode detection through OPENCODE_CLIENT is telemetry-only, never an installation target. Dot-prefixed temporary names must not match skill files, failures preserve the original error, and top-level .agents/.claude roots must not be created merely for detection because their presence signals an installed agent.