feat: add pi-native Signum runtime MVP

CHANGELOG.md

@@ -2,6 +2,18 @@
 
 ## [Unreleased]
 
+### Added
+- pi-native Signum runtime MVP from this repo root
+  - root `package.json` now exposes the repo as a pi-installable package with `pi.extensions`
+  - native `/signum` command for pi under `platforms/pi/`
+  - pi runtime support for `explain`, `init --harness`, `archive`, `close`, and `/signum <task>`
+  - pi task path now covers CONTRACT -> EXECUTE -> AUDIT -> PACK
+  - runtime policy-wrapped engineer tools, execute receipts, audit artifacts, and proofpack generation
+  - `tests/test-pi-extension.sh` plus optional live smoke `tests/test-pi-full-pipeline.sh`
+
+### Notes
+- pi AUDIT now supports bounded iterative repair parity with `.signum/audit_iteration_log.json`, `.signum/repair_brief.json`, per-pass snapshots under `.signum/iterations/`, and proofpack `iterativeAudit` summaries
+
 ## [4.19.1] - 2026-04-20
 
 ### Fixed

README.md

@@ -32,6 +32,8 @@ AI can generate a function in seconds; telling you whether it is correct takes l
 
 ## Install
 
+### Claude Code
+
 <!-- INSTALL:START -->
 ```bash
 claude plugin marketplace add heurema/emporium
@@ -50,13 +52,45 @@ claude plugin install .
 
 </details>
 
+### pi
+
+Signum also ships a native pi runtime from this same repo. The root package is the install surface.
+
+```bash
+pi install . -l
+```
+
+Then run inside pi:
+
+```text
+/signum explain
+/signum init --harness
+/signum your task here
+```
+
+For local non-interactive smoke work only, you may use:
+
+```bash
+SIGNUM_PI_AUTO_APPROVE=1 pi --no-extensions -e ./platforms/pi/extensions/signum/index.ts --mode json --no-session '/signum your task here'
+```
+
+`SIGNUM_PI_AUTO_APPROVE=1` is development-only and not the normal approval UX.
+
 ## Quick start
 
+### Claude Code
+
 ```bash
 # Run — describe what you want to build
 /signum "your task description"
 ```
 
+### pi
+
+```text
+/signum your task description
+```
+
 Signum grades your spec, shows the contract for approval, implements with an automatic repair loop, audits from multiple angles, and produces `proofpack.json` plus an advisory `anti_entropy_report.json`.
 
 Storage model:

docs/PLANS.md

@@ -25,6 +25,7 @@ review_cadence: monthly
 | Root anti-entropy / RECONCILE design | `docs/plans/2026-04-10-root-anti-entropy-reconcile-design.md` | active | Current recommendation: report-only anti-entropy first, no root Phase 5 mutation |
 | Thin CLI extraction | `docs/thin-cli-extraction-plan.md` | active | Tracks deterministic-core extraction to Rust / `signum-core` |
 | Iterative audit behavior | `docs/plans/2026-03-15-iterative-audit-design.md` | active | Design reference for review/fix loop behavior |
+| pi native integration | `docs/plans/2026-04-20-pi-native-integration-plan.md` | active | MVP shipped in repo overlay; current follow-up is docs, tests, and release hardening |
 
 ## Recent Resolved Planning Debt
 - Canonical source policy is now documented in `docs/reference.md`.
@@ -38,6 +39,7 @@ review_cadence: monthly
 - Implement the first report-only anti-entropy artifact without changing the canonical root phase model.
 - Extend thin-cli planning from extraction inventory to a stable protocol/event model for `signum-core`.
 - Decide whether `README.md` and `CHANGELOG.md` need an immediate sync pass for `init --harness`.
+- Finish Slice 6 for pi-native `/signum`: root docs sync, full-pipeline smoke coverage, and release hardening.
 
 ## Archive Rules
 - Keep active planning docs under `docs/plans/`.