fix(kernel): require verified caller identity for the Secret Service bridge

[Antriksh1984]

Summary

SecretServiceBridge proxies the password vault to out-of-process plugins over the kernel's gRPC IPC server - getUserSecrets/getUserSecretsWithSharingInfo return plaintext passwords and TOTP recovery codes, and shareSecret/unshareSecret grant or revoke another user's or role's access to a secret. Continuing the BossConsole#53 caller-identity work, none of this bridge's nine RPCs checked who was calling before this: any process able to open a connection to the kernel IPC server - not only the plugins the host itself loaded - could read or reshare every secret the signed-in user owns.

• Gate every RPC (GetUserSecrets, GetUserSecretsWithSharingInfo, SearchSecrets, CreateSecret, UpdateSecret, DeleteSecret, GetSecretShares, ShareSecret, UnshareSecret) behind authenticatedCallerOrRefuse(), the same PERMISSION_DENIED-on-no-verified-identity pattern PluginUIServiceBridge established for Remote UI: StreamUI has no plugin identity check — one plugin can bind another's surface #53.
• A call with no ProcessIdentityInterceptor-verified identity is refused before it ever reaches SecretDataProvider, so a refusal can't leak timing/existence information either.

Test plan

• New SecretServiceBridgeTest runs a real gRPC server behind ProcessIdentityInterceptor with a real client stub (no mocking of the transport): asserts every RPC is refused with PERMISSION_DENIED for an anonymous caller and that the fake vault provider is never invoked, and that an authenticated caller can still read, create, share, unshare and delete secrets normally.
• ./gradlew :composeApp:compileKotlinDesktop :composeApp:compileTestKotlinDesktop
• ./gradlew :composeApp:desktopTest --tests "ai.rever.boss.kernel.services.SecretServiceBridgeTest"
• ./gradlew :composeApp:ktlintDesktopTestSourceSetCheck
• ./gradlew :composeApp:detektDesktopMain :composeApp:detektDesktopTest

[kshivang]

@Antriksh1984 Thank you for the Secret Service caller-identity fix. I compared the actual production and test diffs with your earlier #505 at 7bf4f6e0db504495cfebf6e76ba552933d2cce0c.

#505 already covers all nine RPCs in #598: list, list-with-sharing, search, create, update, delete, get shares, share and unshare. Each gate uses the same interceptor-verified identity and refuses before provider access. #505 also preserves mutation attribution and RBAC gates, with explicit anonymous refusal tests for every vault method plus revoked-token and late-registration coverage. The additional positive workflow assertions in #598 exercise unchanged provider mapping; there is no additional production fix to port. Both heads have green CI; #505's prior focused tests and quality checks are recorded in its review thread.

Closing #598 as fully superseded by #505, keeping the older and more complete implementation. The surviving description explicitly credits you, Antriksh (@Antriksh1984), for both submissions and distinguishes previous reviewer repairs. #505 remains open against dev; neither PR is being merged or auto-merged. #550's Supabase/download guards and #575's transport/ownership changes are distinct and are not included in this closure.