chore: Upgrade oclif packages and pin Node to v20.19.0

[Copilot]

Addresses Dependabot advisory #24 (minimatch vulnerability across the oclif dependency tree). Also resolves a Node engine conflict introduced by eslint-config-oclif@6, which nests eslint@10 requiring ^20.19.0.

Dependency changes

Package	From	To	Type
@oclif/core	^4	^4.8.1	dep
@oclif/plugin-help	^6	^6.2.37	dep
@oclif/plugin-warn-if-update-available	^3.1.46	^3.1.55	dep
eslint-config-oclif	^5	^6	devDep
eslint-config-oclif-typescript	^2	^3	devDep
oclif	^4	^4.22.81	devDep

Node version

.nvmrc bumped from v20.17.0 → v20.19.0 to satisfy the ^20.19.0 || ^22.13.0 || >=24 engine requirement from eslint-config-oclif@6's nested eslint@10 dependency.

Known limitation

eslint-config-oclif has a self-dependency on an earlier version of itself (tracked in oclif/eslint-config-oclif#319), so npm audit still reports issues from that chain. Production deps are clean:

$ npm audit --omit dev
found 0 vulnerabilities

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