SendMessage/ListAgents cross-session peer messaging broken after CCD 2.1.258 -> 2.1.260 update (Windows desktop app)

**Environment**
- Claude desktop app (Windows), CCD version 2.1.260, app version 1.46388.x
- Platform: win32, Windows 11 Pro

**What broke**
Cross-session `SendMessage`/`ListAgents` peer-messaging (used to relay messages between
multiple concurrent Claude Code sessions) stopped working after a background app
auto-update from CCD 2.1.258 to 2.1.260 (observed 2026-09-04 13:58-13:59 KST via
%LOCALAPPDATA%\Claude\logs\main.log, app version 1.44121.4 -> 1.46388.1).

**Reproduction / evidence**
- A long-running session was actively using `SendMessage` successfully while on CCD
2.1.258.
- The desktop app auto-updated in the background while that session stayed open (no
visible restart of the conversation).
- Immediately after, `SendMessage` calls in that same session began failing, and
`ListAgents` started reporting the session's own peer identity as an auto-generated
name (e.g. `engine-78`) with a different internal session ID, instead of its
previously-fixed name (set via `/rename `).
- Every session created since that update (still on CCD 2.1.260) starts with
`SendMessage` unavailable from its very first message -- this is not limited to
sessions that were open during the update.
- Re-running `/rename ` does not restore it. The user verbally confirming session
identity in chat does not restore it either.
- This is not a platform (native-Windows-vs-WSL2/macOS/Linux) limitation: the same
session, in the same desktop app, had `SendMessage` working before the update.
- Not related to long-session context compaction: reproduces in sessions only minutes
old.
- Possibly related: `~/.claude.json`'s cached GrowthBook flag `tengu_team_discovery` is
`false` on the affected machine as of 2026-09-05 (not independently confirmed as the
cause).

**Impact**
Breaks any workflow relying on multiple Claude Code sessions coordinating via
SendMessage/ListAgents (e.g. a supervisor/executor/verifier multi-agent setup).

**Request**
Please confirm whether this is a known regression in 2.1.259/2.1.260, and if so, an ETA
for a fix or a way to roll back to 2.1.258 behavior.

Start by comparing the affected CCD 2.1.258 and 2.1.260 behavior using the reproduction in the issue, then inspect %LOCALAPPDATA%\Claude\logs\main.log and the ~/.claude.json tengu_team_discovery flag. Reproduce SendMessage and ListAgents across concurrent sessions; done means identifying the regression and documenting a confirmed fix or rollback path.

Written by the indexing model from the issue text.