feat: schema export + preset snapshot tests (#72, #80)

[alohays]

Summary

• forge schema command (feat: forge schema command (Zod to JSON Schema introspection) #72): Outputs full JSON Schema derived from MonitorForgeConfigSchema via zod-to-json-schema. Supports forge schema <section> to filter by sub-schema (e.g., sources, panels, map). Includes _note field documenting that Zod .refine() validators are not represented in JSON Schema output.
• Preset snapshot tests (test: preset validation snapshot tests #80): Validates all 15 preset JSON files parse correctly through MonitorForgeConfigSchema.parse() and snapshots the results. Catches schema drift and preset corruption by verifying version and _meta fields on every preset.
• Adds zod-to-json-schema as a dev dependency.

Verified

• forge schema outputs valid JSON Schema with version field
• forge schema sources correctly filters to the sources sub-schema
• All 15 preset files pass snapshot validation (46 tests)
• npx vitest run forge/ passes (405 tests, 0 failures)
• npx tsc --noEmit has zero errors

Test plan

• npx tsx forge/bin/forge.ts schema --format json outputs full JSON Schema
• npx tsx forge/bin/forge.ts schema sources --format json outputs sources sub-schema only
• npx tsx forge/bin/forge.ts schema invalid-section returns structured error
• npx vitest run forge/src/presets passes all snapshot tests
• CI pipeline passes

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