docs: add v0.4.0 release notes

docs/release-notes/v0.4.0.md

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+## BotWire v0.4.0 — RedactWire PII detection + clear block messaging
+
+This release replaces BotWire's built-in PII guard with [RedactWire](https://github.com/adamy/RedactWire), and makes a blocked message tell the user *why* it was refused instead of showing a generic error. Defaults stay on; `AdditionalPatterns` keeps working.
+
+### New features
+
+- **RedactWire-backed PII detection** — the old 4-pattern regex guard is replaced by RedactWire 0.4.0: invariant rules (email, credit card with Luhn, IPv4, IBAN with mod-97) plus secret/credential detection (API keys, tokens), evaluated against the default culture (`CultureInfo.CurrentCulture`). Checksum validation means far fewer false positives than a bare regex.
+
+- **`PiiGuardOptions.ConfigureDetector`** — an `Action<PiiDetectorBuilder>` hook to configure RedactWire directly: enable country rule packs (`AddCulture`), add custom `IPiiRule`s, or pick an overlap strategy.
+
+  ```csharp
+  builder.Services.AddBotWire(opts =>
+  {
+      opts.PiiGuard.ConfigureDetector = b => b.AddCulture(new CultureInfo("en-US"));
+  });
+  ```
+
+- **Clear PII messaging** — a blocked message now returns a distinct **`PiiBlocked`** status carrying the configurable `PiiGuardOptions.RejectionMessage`. The embedded widget and the `botwire-js` client surface that message instead of the generic "Something went wrong", so users know their message contained personal data.
+
+- **Dedicated prompt-injection message** — injection blocks no longer reuse the PII copy; they get their own neutral `PromptInjectionOptions.RejectionMessage`.
+
+### Breaking changes
+
+- **PII blocks now return status `PiiBlocked`** (previously `Blocked`). Clients that branch on `Blocked` to detect a PII refusal should add `PiiBlocked`.
+- **Stronger, broader detection.** Messages containing IBANs, IP addresses, or secrets/API keys are now blocked where the old four-pattern guard let them through. Tune via `ConfigureDetector` or adjust `RejectionMessage`.
+- **`PiiGuardOptions` exposes RedactWire's `PiiDetectorBuilder`**, so RedactWire is now a public dependency of `BotWire.Core`. (Its base package does not pull `System.Text.Json` — structured scanning lives in a separate add-on — so there is no net-new transitive dependency.)
+
+`AdditionalPatterns` is unchanged: custom regex patterns still work, now bridged to RedactWire custom rules and compiled with a 100 ms match timeout to bound ReDoS.
+
+### Fixes
+
+- The embedded `widget.js` is now served with `Cache-Control: no-cache` and a content-hash **ETag** instead of a one-hour `max-age`. A rebuilt widget reaches browsers immediately (304 when unchanged) rather than being stuck behind a stale cache.
+
+### Get started
+
+```
+dotnet add package BotWire.AspNetCore
+```
+
+See the [README](https://github.com/adamy/BotWire#readme) for the quick start and `PiiGuardOptions` for PII configuration.