feat: add dsh-redact plugin

[kestiny18]

What

• add dsh-redact@0.1.0, a fail-closed DeepSeek Harness canonical tool-output tokenization plugin
• keep reversible secret mappings Agent-scoped and in memory
• expose restoration only as an explicit trusted same-process presentation primitive
• add architecture/design documentation, release verification, and adversarial Harness integration tests

Why

Prevent common credentials in tool results from entering model context or durable Session history while preserving output-schema validation and deliberately avoiding token restoration into tool arguments.

Impact

• new independently published package: dsh-redact
• no runtime changes to dsh-usage
• no long-lived npm credentials; publication remains GitHub Release + npm OIDC

Checks

• pnpm install --frozen-lockfile
• pnpm --filter dsh-redact run release:check
• pnpm run check
• pnpm run pack:check
• git diff --check
• pnpm audit --audit-level high