fix(security): harden triage command dispatch

[bokelley]

Summary

• replace shell interpolation of /triage dispatch payloads with a fail-closed Node parser that reads GITHUB_EVENT_PATH
• validate event/action shape, positive IDs, actors, command, and the documented empty/execute/clarify/defer modifiers before publishing any step output
• prevent forged dispatch payloads from redirecting the reaction PAT to another repository
• add exact-parser regression coverage for shell substitution, metacharacters, output injection, flags, Unicode confusables, invalid actors, and valid GitHub event variants

Security finding addressed

• codex-security/triage-slash-args-shell-injection — slash-command arguments were compiled directly into Bash in a secret-bearing workflow

Validation

• focused adversarial tests: 46/46 passed
• actionlint / embedded ShellCheck: passed
• server TypeScript no-emit check: passed
• git diff --check, protocol-scope, package/lock, and dist guards: passed
• independent security and testing expert reviews: CLEAN

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