chore(release): 0.3.0 — a gate-loosening change earns a minor, not a patch

Version-sync across pyproject, __init__, README badge/status/install pin and
CHANGELOG, and cut [Unreleased] to [0.3.0].

Why minor: 0772daf made allow_reserved default to True on okf.import_bundle,
so a consumer who upgrades without touching their code gets a LOOSENED gate --
a reserved index.md/log.md in a received bundle is now scanned and may become
mergeable, where v0.2.0 rejected it unconditionally. Under 0.x a >=0.2,<0.3
pin absorbs a 0.2.1 silently but stops at 0.3.0, which is the signal such a
change should send. Found by llm-ingestion-okf, our first real downstream
consumer, against main.

Development Status stays 3 - Alpha and the README keeps "the public API may
still change": this ships the hardened surface (Sessions A/A2/B) to consumers
who are still pinned at v0.2.0 and therefore have none of it. It is not the
v1.0 freeze -- that stays gated on the first real integration coming back
green, which is exactly what 0.3.0 makes possible.

Verified: 522 passed; coverage matrix exit 0; version string present in all
four files; no dangling 0.2.0 outside CHANGELOG history.

CHANGELOG.md

@@ -7,9 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-> **The next release is 0.3.0, not 0.2.1.** The changes in *Changed* below alter what
-> an existing caller observes without any code change on their side. A `>=0.2,<0.3`
-> pin therefore stops here deliberately — re-test that branch before widening it.
+## [0.3.0] — 2026-07-25
+
+> **A minor bump, not a patch — deliberately.** The changes under *Changed* alter what
+> an existing caller observes with no code change on their side, so a `>=0.2,<0.3` pin
+> stops here rather than absorbing them silently. Re-test that branch before widening
+> the pin. Still alpha: the public API may change again before 1.0.
 
 ### Changed — observable gate behaviour (re-test before upgrading)