chore(release): v0.31.0 (M4 API/format freeze) (#156)

Bump packages/{basou,core,cli,sdk} to 0.31.0 and finalize the CHANGELOG
section. Bundles the M4 (API/format freeze) pass:

- Portfolio view live repo links (#152).
- .basou format version gate: forward-compatible format major (#153).
- Remove project.repository_url; --repo-url kept as a deprecated no-op (#154).
- semver 1.0 compatibility and stability policy doc (#155).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: takashi-matsuyama

CHANGELOG.md

@@ -3,7 +3,7 @@
 All notable changes to **basou** are recorded here. The project follows
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html) starting with v0.1.0.
 
-## Unreleased
+## 0.31.0 — 2026-07-02
 
 ### Removed
 

packages/basou/package.json

@@ -1,6 +1,6 @@
 {
   "name": "basou",
-  "version": "0.30.0",
+  "version": "0.31.0",
   "type": "module",
   "license": "Apache-2.0",
   "description": "Convenience installer for the @basou/cli command. `npm install -g basou` exposes the same `basou` binary as `npm install -g @basou/cli`.",

packages/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@basou/cli",
-  "version": "0.30.0",
+  "version": "0.31.0",
   "type": "module",
   "license": "Apache-2.0",
   "description": "basou command-line tool",

packages/core/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@basou/core",
-  "version": "0.30.0",
+  "version": "0.31.0",
   "type": "module",
   "license": "Apache-2.0",
   "description": "Core primitives for Basou: sessions, events, approvals, git capability",

packages/sdk/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@basou/sdk",
-  "version": "0.30.0",
+  "version": "0.31.0",
   "type": "module",
   "license": "Apache-2.0",
   "description": "Read-only SDK for Basou: a stable, ergonomic API to read a workspace's provenance (sessions, events, tasks, approvals, stats).",