Antigravity conversion overwrites commands with colliding normalized names

At 9ac32720, converting commands named review.docs and review-docs emits two commands named review-docs. Writing the bundle leaves only .agy/commands/review-docs.toml, containing the second prompt.

Reproduce with a Claude plugin containing those two command names, distinct prompt bodies, and no agents or skills; run convertClaudeToAntigravity followed by writeAntigravityBundle into a temporary directory.

Expected: both prompts survive in distinct command files, following the existing uniqueName() suffix convention. Actual: the first prompt is overwritten.

convertCommand() calls uniqueName(pathKey, usedNames) but discards its return value. Use that returned path and add converter-to-writer regression coverage for flat/namespaced commands and occupied numeric suffixes. Scope: command-name collisions only; no skill-directory or path-security changes.