fix(cli): resolve relative media timing in snapshots

[miguel-heygen]

Summary

• use the runtime clip manifest when selecting and seeking video frames for snapshots
• preserve numeric authored timing as a fallback when no manifest entry is available
• add a regression test for relative starts that reference hyphenated clip IDs

Test plan

• bun test packages/cli/src/commands/snapshot.test.ts — 17 passed
• bunx oxlint packages/cli/src/commands/snapshot.ts packages/cli/src/commands/snapshot.test.ts — clean
• bunx oxfmt --check packages/cli/src/commands/snapshot.ts packages/cli/src/commands/snapshot.test.ts — clean
• git diff --check — clean

Broader CLI tests and typecheck were attempted, but this isolated worktree lacks the generated workspace packages/dev dependencies required by the existing suite.