feat: rewrite as thin wrapper around supermodel CLI

Replaces the direct API client (TypeScript SDK) with the Supermodel CLI
binary. The action now installs the CLI on the runner and delegates all
analysis to `supermodel dead-code -o json`, then handles GitHub-specific
concerns (PR diff scoping, PR comment, action outputs, fail-on-dead-code).

Feature parity maintained:
- comment-on-pr, fail-on-dead-code, ignore-patterns, timeout-seconds unchanged
- PR diff scoping (scope findings to changed files) unchanged

New inputs:
- min-confidence: pass --min-confidence to CLI (high/medium/low)
- cli-version: pin the CLI version to install (default: latest)

Removed:
- @supermodeltools/sdk dependency (analysis now done by CLI)
- minimatch dependency (glob filtering now done by CLI --ignore flag)
- filterByIgnorePatterns (logic moved to CLI)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

feat: rewrite as thin wrapper around supermodel CLI

Replaces the direct API client (TypeScript SDK) with the Supermodel CLI
binary. The action now installs the CLI on the runner and delegates all
analysis to `supermodel dead-code -o json`, then handles GitHub-specific
concerns (PR diff scoping, PR comment, action outputs, fail-on-dead-code).

Feature parity maintained:
- comment-on-pr, fail-on-dead-code, ignore-patterns, timeout-seconds unchanged
- PR diff scoping (scope findings to changed files) unchanged

New inputs:
- min-confidence: pass --min-confidence to CLI (high/medium/low)
- cli-version: pin the CLI version to install (default: latest)

Removed:
- @supermodeltools/sdk dependency (analysis now done by CLI)
- minimatch dependency (glob filtering now done by CLI --ignore flag)
- filterByIgnorePatterns (logic moved to CLI)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fix: Use UUID for idempotency key to scale to concurrent users (#4)