Facts speaks Facter’s input/output contract, not Puppet’s runtime behavior

Facts is a fact-discovery library and CLI that implements Facter’s input and output contract — the shapes of facts in and out — but it is not bound to Puppet or to what Puppet does at runtime. The --puppet (-p) flag was the one feature that crossed that line: it appended Puppet’s agent plugin-fact destination (vardir/facts.d, e.g. /opt/puppetlabs/puppet/cache/facts.d) to the external-fact directories, and it warned when Puppet had pluginsync’d Ruby custom facts into vardir/lib/facter. Both re-enact what a running puppet agent arranges on a host, not a Facter-shaped input. We remove --puppet, -p, and --no-puppet entirely; they now fail as unknown options. This builds on ADR-0006 (no Ruby DSL) and ADR-0001 (the CLI process edge is the only compatibility boundary).

The boundary this draws, stated so it is not re-crossed by accident:

• In the input contract (kept): Facter’s own external-fact directories, including the ones that live under a puppetlabs/ path — /opt/puppetlabs/facter/facts.d, /etc/puppetlabs/facter/facts.d, ~/.puppetlabs/opt/facter/facts.d — which stay in the always-on default set (internal/engine/config.go). These are Facter’s directories, not Puppet’s runtime; “no Puppet” must not be read as a license to delete them.
• Out of scope (removed): Puppet’s agent-runtime surface — the pluginfactdest/vardir cache (…/puppet/cache/facts.d), pluginsync, and .rb plugin loading.

Puppet’s agent cache is still reachable like any other directory through the explicit external-fact inputs: --external-dir /opt/puppetlabs/puppet/cache/facts.d (or the platform/user equivalent). A .rb file in any external directory is still skipped with a warning by the general loader (ADR-0006); only the --puppet-specific warning about vardir/lib/facter is gone, because that directory was never loaded as an external dir to begin with. The inert EngineConfig.Puppet field and internal/engine/puppet.go are deleted, so the engine carries no notion of Puppet.

Considered Options

• Keep --puppet as a convenience bridge — rejected: it is the only feature that reaches into Puppet’s runtime rather than Facter’s contract, which is precisely the coupling this project no longer wants. Auto-discovering an agent’s on-disk cache is Puppet-runtime behavior, not a Facter input shape.
• Hide the CLI flag but keep the engine helper — rejected: the EngineConfig.Puppet field was already inert (set by the CLI, read by no engine code) and PuppetPluginFactDirs was CLI-only; a library that claims no Puppet coupling should not ship a Puppet path-resolver. Full delete to the engine.
• Emit a bespoke --puppet is removed; use --external-dir error — rejected: the validator already fails unknown flags with unrecognised option '--puppet'; a special-cased hint for one flag is inconsistent with every other removed flag (ADR-0006’s --custom-dir, ADR-0007’s --show-legacy) and keeps a Puppet-shaped stub. The migration guidance lives in the CHANGELOG.
• Remove --puppet (chosen) — breaking only on paper; the project is pre-1.0 (v0.0.x, no compatibility guarantee yet), so the input-contract amendment is cheap now and expensive after 1.0. Facter-configured hosts are unaffected — their facts.d directories are Facter’s, which we keep.