analyze: exit code is 0 even when nothing was successfully processed #115

The analyze command returns exit code 0 unconditionally once it runs, regardless of what happened to the input files. Two observed cases:

• Analyzing a Player build without TypeTrees: every file is skipped (Successfully processed files: 0, Failed files: 0, Files without TypeTrees: 4), a fully-formed but empty database is written, and the exit code is 0. This is a total failure presented as success — a script (or AI agent) that checks the exit code, or checks that the .db file exists, will conclude the analysis worked.
• A run with Failed files: 8 (mixed input) also exits 0.

The per-file skip reasons do go to stderr, which is good, but detecting "this run produced nothing useful" currently requires parsing the summary sentence on stdout.

Proposal:

• Return a non-zero exit code when Successfully processed files == 0 (nothing usable was analyzed).
• Decide what partial failure (Failed files > 0 but some successes) should do — a distinct non-zero code, or keep 0 and rely on the stderr lines. Open for discussion; the zero-successes case is the important one.

For comparison, dump and find-refs already return 1 on their failure cases.

Found during a review of how well AI agents can use the tool unattended; the exit code is the main signal such tools rely on.