ci: trigger nightly on main merges and add CLI verbose diagnostics

[oscarcv]

What changed:

1. Nightly on each merge to main + tap update

• Updated .github/workflows/nightly.yml:
  • Trigger is now PR close on main and runs only when merged.
  • Keeps manual trigger (workflow_dispatch).
  • Still publishes nightly assets and updates snapshot-report-nightly formula in oscarcv/tap.
  • Nightly release body now references merge commit SHA when available.

2. Verbose diagnostics for report generation

• Updated Sources/snapshot-report/CLI/main.swift:
  • Added --verbose / -v CLI flag.
  • Verbose mode shows detailed phase diagnostics:
    • input resolution counts
    • merge summary counts
    • odiff detection/path usage
    • per-phase progress
    • total processing time
  • Default mode stays concise (final success output).

3. Documentation

• Updated README.md:
  • Added nightly automation behavior (on merge to main + tap nightly formula).
  • Added --verbose usage example for diagnostics