0001. Record architecture decisions

Context

pdca-harness has made decisions since its first commit (2026-06-04) that everything else depends on: file-derived state, a human-only accept, a decorrelated reviewer. None of them is written down outside commit messages and docstrings. The result is visible in the tracker: #505 (restructure outline) and #607 (state read back from Markdown) both re-derive decisions the code already embodies, and docstrings carry issue history at more than one line of prose per line of code. The approach review of 2026-10-15 named this as a gap.

Decision

We will record significant decisions as Architecture Decision Records in docs/design/adr/, using the Nygard-style template in docs/design/templates/adr.md.

• Files are numbered NNNN-short-title.md and indexed in adr/README.md.
• An ADR starts as Proposed. Once Accepted it MUST NOT be edited; a later ADR supersedes it, and the index records the supersession.
• A decision is "significant" when reversing it would change what the harness guarantees (the accept path, decorrelation, the evidence rule), the shape of the bundle files, or the contract with rendered instances.
• Decisions made before this ADR are backfilled (0002–0008), each dated to the commit or issue that made it.

Consequences

The reasons for the driver's shape survive refactors, and a migration step can cite the decision it changes. Reopening a settled question needs a new ADR, which costs more than an issue comment; that is intended. Docstrings can drop their issue history over time and point at an ADR instead.