feat(desktop): add app-owned conversation reader

[timigod]

Summary

• add a credentialless buzz-read messages client for bounded programmatic conversation reads
• perform authenticated relay queries inside the already-running signed Buzz Desktop process through an owner-only Unix socket, so the private key never crosses the process boundary
• require the production bundle identifier, Block Developer ID signature and Team ID, a keyring-backed current identity, the connected Desktop PID, and unchanged socket/identity/workspace scope through response completion
• reject stale, replayed, oversized, malformed, out-of-scope, or identity-changing reads and return only bounded, redacted message projections
• bundle the Unix-only sidecar through the existing signed release and canary workflows without changing the normal buzz CLI or message-reply behavior

Related issue

Closest related work: #5884. That PR explicitly lets the CLI read the Desktop secret after opt-in. This PR serves the narrower operator-read use case without exporting the private key from the signed Desktop process, so it is complementary rather than a duplicate.

Testing

• cargo test --manifest-path desktop/src-tauri/Cargo.toml --target-dir target operator_read (18 passed)
• cargo test --manifest-path desktop/src-tauri/Cargo.toml --target-dir target tauri_platform_configs_bundle_unix_only_sidecars_on_supported_hosts
• scripts/test-buzz-read-packaging.sh (copies real fixture binaries through the bundler)
• scripts/test-signed-canary-contract.sh
• CHECK_FILE_SIZES_BASE=00e61eafa917d296104006576b7a2ddbfd58bb5a node desktop/scripts/check-file-sizes.mjs
• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml --all -- --check
• cargo clippy --manifest-path desktop/src-tauri/Cargo.toml --target-dir target --lib --bin buzz-read -- -D warnings
• git diff --check
• independent security/correctness review of exact head a033e8a97d67182a23e3e2a312b5ac35e83b40fc: GO, no MUST_FIX findings

This has no UI change. A live production read is deliberately deferred until the commit is merged and available in a Block-signed distribution; no ad-hoc or unsigned Desktop build was installed.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 00e61eafa917d296104006576b7a2ddbfd58bb5a...a033e8a97d67182a23e3e2a312b5ac35e83b40fc.
A new review must complete for this exact range. When manual authorization
is required, a Block organization member must comment exactly
@buzz-security-review a033e8a97d67182a23e3e2a312b5ac35e83b40fc to authorize a new review.
Any previous review applies only to its recorded range.

[timigod]

@block/buzz-oss-team Could a Block member unblock the current exact head in two places?

1. Authorize the SHA-bound security review with this exact command:
   @buzz-security-review a033e8a97d67182a23e3e2a312b5ac35e83b40fc
2. Approve the pending first-run fork workflows for CI and Desktop Release Candidate.

The branch is clean, DCO and zizmor pass, focused/security checks are documented in the PR, and no unsigned build has been installed.

[timigod]

Local verification update for exact head a033e8a:

• CHECK_FILE_SIZES_BASE=upstream/main just ci passed formatting, file-size checks, workspace/Desktop/Tauri/Web/Flutter static checks, the reader-specific tests, and the reader/compatibility tests reached during the full suite.
• The full run stopped in the unchanged upstream test managed_agents::discovery::bounded_command::tests::returns_when_escaped_descendant_retains_pipe after 3,022 Desktop tests passed (1 failed, 18 ignored). The failure is an empty descendant-PID parse at bounded_command.rs:705. An isolated exact rerun reproduced it. This branch has no diff from upstream/main in that file.
• All 18 focused reader tests are green, including signed-peer validation, PID-bound receipts, replay/freshness rejection, identity-generation A-B-A fencing, bounds/redaction, and unchanged CLI/depth-one reply behavior.
• GitHub CI and Desktop Release Candidate remain action_required with zero jobs until a Block maintainer approves the fork workflows. The SHA-bound security review also still needs the exact authorization already requested above.

No unsigned build has been installed and no live identity or Keychain state has been touched.