feat: --fail-on, --quiet, exit code 2, rules and explain (#25)

[hermanngeorge15]

Covers CLI-06 and CLI-07 from #25. Depended on #21 — with everything graded CRITICAL or HIGH there was nothing for --fail-on to threshold on, which is why that issue lists this as blocked.

Exit codes are the real interface to CI

0  no findings
1  findings at or above --fail-on
2  findings, all below --fail-on

The third one is the point. Without it, --fail-on critical would exit 0 on a file full of HIGH findings, and any pipeline checking for zero would call it clean — the flag would quietly become a way to hide things rather than a way to prioritise them.

Verified as a matrix:

document	--fail-on critical	high	medium	low
CRITICAL payload	1	1	1	1
MEDIUM payload	2	2	1	1
clean	0	0	0	0

--fail-on raises the bar for failing, not for reporting. Findings below it are still printed, because a user who can't see them can't judge whether the bar is set right. Pinned by its own test.

rules and explain

The question a blocked commit creates:

$ injection-scanner rules
ID       SEVERITY  CATEGORY               NAME
PI001    CRITICAL  role_override          ignore-previous-instructions
PI003    MEDIUM    role_override          you-are-now
PI041    LOW       encoding               zero-width-chars

$ injection-scanner explain pi035
PI035  jailbreak-prompt  [LOW]
Category:    jailbreak
Detects:     Explicit jailbreak prompt reference
Remediation: Remove jailbreak prompt.
Pattern:     (?i)\bjailbreak\s+prompt\b

Suppress one occurrence with:
  <!-- injection-scanner:ignore PI035 -->

Both show the effective severity, resolved against the category default — a listing with a blank for every pattern that inherits would be worse than none. explain is case-insensitive, ends with the exact suppression directive, and on an unknown id suggests nearby ones instead of just failing. rules --format json is machine-readable.

Two bugs worth naming

{:<9} silently did nothing to the severity column. A width specifier only pads a custom Display impl if that impl routes through Formatter::pad — this one doesn't, so every column ran together. Rendered to a String first.

The new items landed between #[derive(Subcommand)] and enum Commands, quietly reattaching the derive to the wrong type. Every #[arg] in the file then failed to resolve, with an error pointing at lines I hadn't touched.

Tests

7 new, including the full exit-code matrix and the below-threshold-still-reported guarantee. Full suite 23 binaries green, clippy clean.

Not in this PR: --baseline (CLI-08), --no-color, --stdin-name. --baseline is the larger adoption feature and deserves its own change.