feat: keep thread replies one level deep across clients

[KanjiTheryo]

Summary

Keep conversations readable by rendering all thread responses in one chronological list on desktop and mobile. Replying to a response continues the original thread and retains a clickable reference to the selected response.

• SDK/CLI, native desktop/managed-agent, mobile, ACP typing, and workflow producers target the original root before signing. A reply-context tag preserves the selected response without adding nesting.
• Historical signed events remain unchanged. Flat rendering includes deep descendants and missing intermediate parents; drafts retain selected-response context and separate legacy draft identities.
• Add opt-in relay enforcement through BUZZ_SINGLE_LEVEL_REPLY_COMMUNITIES (explicit UUIDs; empty/off by default). Validate context ancestry and reject new deeper replies for enabled communities. Upgrade and verify clients/agents before enabling enforcement; disable enforcement first during rollback.

Related issue

Related: #2415, #3799, #4072. This intentionally offers a flat conversation model as an alternative to deeper nesting discussed in #2851; selected-response context remains available. Related open UI work includes #7036 and #6695. Maintainer agreement on this cross-client behavior is required before merge.

Testing

• Full local just ci passed at c1c01bcc1169895d839fafa3c06bd778abcc53dc: formatting, lint/static checks, Rust/agent/native tests, 6,513 frontend tests, desktop/web builds, and 2,130 mobile tests.
• The targeted messages/channels run also passed 1,891 tests. The full run caught a legacy ACP recovery-notice assertion; it now requires root parentage and retained response context, and the corrected agent suite passes.
• Prior feature verification: 306 SDK tests; 38 focused mobile tests; 22 native message-command tests; reply-specific native tests; targeted draft tests; TypeScript/frontend build; two browser journeys including draft close/reopen and reply-context navigation.
• Isolated PostgreSQL/workflow integration and a live local CLI-to-relay check verified signed root/context tags, depth 1, policy-off compatibility, policy-on rejection, and unchanged historical signed events. Synthetic local identities/data only.
• Upstream CI, Desktop Release Candidate, and Docker image workflows currently require maintainer approval for this fork. DCO passed on the initial commit; the follow-up commit also includes DCO sign-off. No official desktop/mobile package or hosted relay was released.
• Screenshots are not attached yet: the required scripts/post-screenshots.sh publishes to an upstream branch, and this contributor account has read-only upstream access. Local browser captures exist for maintainer-assisted attachment.

Ready for maintainer review. Next action: an upstream maintainer approves the fork workflows and reviews the cross-client behavior, then the release/community operators coordinate client/agent upgrades before enabling enforcement for the intended community. This PR does not activate enforcement by default.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 4cd82f513214aad11c2b742ce7cc7c681e8e32a0...c1c01bcc1169895d839fafa3c06bd778abcc53dc.
A new review must complete for this exact range. When manual authorization
is required, a Block organization member must comment exactly
@buzz-security-review c1c01bcc1169895d839fafa3c06bd778abcc53dc to authorize a new review.
Any previous review applies only to its recorded range.

[KanjiTheryo]

Still want this contributed. It is independent of the ACP DM allowlist work in #7642.

Ready for maintainer review. Fork workflows still need a Block org member to authorize CI. DCO already passed.