chore: retire docs/specs and docs/plans as committed artifacts

Per-cycle specs/plans are step-scoped: every file maps to already-shipped
code, their snippets drift hard against current APIs (InputSpec->PortSpec,
Sim->Harness, schema() removed, C16 zero-dep reversed, ascii-dag retired),
and a stale spec read as an API reference misleads agents into non-compiling
code. No durable knowledge lost — rationale lives in the ledger + code
rustdoc, history in git + the tracker.

- gitignore docs/specs/ and docs/plans/
- delete all 117 existing specs/plans (recoverable via git history)
- CLAUDE.md: specs/plans are local-only ephemeral artifacts

.gitignore

@@ -12,3 +12,7 @@ Cargo.lock.orig
 # `aura sweep` persists run records to ./runs/runs.jsonl by default; that store
 # is local run telemetry, not a repo artifact.
 /runs/
+
+# Specs and plans are kept as local working artifacts, not checked into the repo.
+/docs/specs/
+/docs/plans/