feat(desktop): add protected-build Bestie experiment (#6902)

## Summary

Introduces a protected-build boundary for the default-off Bestie
experiment without adding any Bestie product surface.

- Official OSS builds select an empty protected-feature module and emit
no Bestie/Chief metadata or implementation content.
- Protected internal builds select a separate module graph containing
the Bestie experiment definition.
- Within an internal build, Bestie remains disabled until the user opts
in under Settings → Experiments.
- The production build runs an artifact matrix and fails if OSS output
contains protected content or internal output lacks the Bestie manifest.

## Build contract

| Build variant | User opt-in | Result |
| --- | --- | --- |
| Official OSS | Any/forged | Bestie absent from the compiled artifact |
| Protected internal | Off | Bestie available but disabled |
| Protected internal | On | Bestie enabled |

The companion protected-release change is squareup/buzz-releases#91. It
sets `VITE_BUZZ_BESTIE=1`, requires that exact value, forwards it into
the signed macOS build, and asserts the contract in release validation.

## Why this is separate

This gives later Bestie PRs one build-selected import seam. Protected
implementations must be reachable only from the internal module so they
never enter the official OSS module graph.

## Non-goals

- No Bestie persona or provisioning
- No sidebar, app-chrome, or message-toolbar UI
- No entitlement or secrecy claim: the source is public; this boundary
controls official Block artifacts

## Verification

- Exact commit `523cf49ced03cba9be43836a54d6aa5d6923cc82`
- Full `just ci`: 5,673 Desktop tests, 2,773 Tauri tests, 1,860 mobile
tests, Rust/Tauri/web/mobile static checks and builds
- OSS production artifact: scanner confirms no `Bestie`, `Chief of
Staff`, or `builtin:bestie` content
- Internal production artifact: scanner confirms the protected Bestie
manifest is emitted
- Both build orders verified; `dist` retains the requested variant for
Vite/Tauri packaging

---------

Signed-off-by: Arjun Mahanti <arjun@squareup.com>
Signed-off-by: Fizz <fizz@buzz.local>
Signed-off-by: Fizz <dae5f6af70b8695a8b83c8deae555f63be41630ec2b8cd493e41a439c9527dd8@buzz.block.builderlab.xyz>
Co-authored-by: Codex <noreply@openai.com>
Co-authored-by: Fizz <fizz@buzz.local>
Co-authored-by: Fizz <dae5f6af70b8695a8b83c8deae555f63be41630ec2b8cd493e41a439c9527dd8@buzz.block.builderlab.xyz>

.env.example

@@ -274,6 +274,10 @@ RUST_LOG=buzz_relay=debug,buzz_datastore=info,buzz_db=debug,buzz_auth=debug,buzz
 # app launch while keeping the current identity and relay data.
 # VITE_BUZZ_FORCE_FRESH_ONBOARDING=true
 
+# Protected internal builds only: selects the module graph that contains the
+# default-off Bestie experiment. Official OSS builds must leave this unset.
+# VITE_BUZZ_BESTIE=1
+
 # ── Subscription & filtering ─────────────────────────────────────────────────
 # Subscribe mode: "mentions" (default), "all", or "config" (rule-based).
 # BUZZ_ACP_SUBSCRIBE=mentions