pakka v0.18.0

Consolidation release after the v0.13-v0.17 run. Status line now shows an upgrade segment when a newer plugin version is installed in the local cache beside the running one — pure local readdir, zero network, so silently-stale sessions become visible. Eval suite is clean across all command and agent docs (long-line drift from the week's edits wrapped, rendering identical). DESIGN.md's v0 build plan is marked historical with pointers to living docs, and the week's seven architecture decisions are recorded in the decision log. Assets CI-built with SLSA provenance attestation, SHA256SUMS, CycloneDX SBOMs.