test: add negative and edge-case pass for all input parsers

[rocketman-code]

Context

Parent: #160

Two v0.4.0 bugs were caused by missing negative tests: unknown REPL flags silently ignored (#144) and cut_report returning vacuous-truth direct_import for missing targets (#151). The existing test suite is ~70% happy path.

Expected Behavior

For every input parser and query function, systematically test the failure modes.

Scope

• parse_flags: unknown flags, flags with missing values, duplicate flags, empty input, flags-only (no positional arg)
• Command::parse: unknown commands, empty input, command with only whitespace
• classify_diff_arg: nonexistent files, invalid refs, empty string, special characters
• Session query methods: nonexistent targets, self-referential targets (entry = target), empty graphs
• cut_report: target not in graph, target is entry, target with no importers
• chain_report: unreachable target, target is entry
• parse_size: negative values, overflow, non-numeric, empty
• All error variants: verify Display output and hint() for every Error enum variant