[BUG] Agent Teams: SendMessage from background teammate to team-lead is silently dropped (one-way only)

Bug Description

When a teammate is spawned via Agent() with run_in_background: true and team_name, SendMessage works one-way only:

• ✅ team-lead → teammate: Messages delivered and processed correctly
• ❌ teammate → team-lead: SendMessage({ to: "team-lead" }) returns success on the teammate's end, but the message never appears in the team lead's conversation. Silently dropped.

Reproduction Steps

1. Team lead spawns a background teammate:

``js
Agent({
name: "Venus",
team_name: "my-team",
run_in_background: true,
mode: "bypassPermissions",
prompt: "Do some work, then report back via SendMessage"
})
// Returns: name: Venus-19, team_name: my-team
``

1. Team lead sends a message to the teammate — works fine:

``js
SendMessage({ to: "Venus-19", summary: "status check", message: "What's your progress?" })
// Returns: {"success": true, "message": "Message sent to Venus-19's inbox"}
``
Venus-19 receives and processes this message.

1. Teammate sends a message back — silently dropped:

``js
// Venus-19 calls:
SendMessage({ to: "team-lead", summary: "status update", message: "All fixes done, pushed commit abc123" })
// Venus-19 sees success on her end
``
The team lead never receives this message. Nothing appears in the conversation, no notification, no error.

1. Multiple messages sent by the teammate over ~10 minutes — none delivered.

Expected Behavior

SendMessage({ to: "team-lead" }) from a background teammate should deliver the message to the team lead's conversation, the same way messages flow in the other direction.

Actual Behavior

The teammate's SendMessage call returns success, but the message is silently dropped. The team lead has no way to know the teammate sent anything.

Impact

• Team lead assumes teammate is dead → spawns unnecessary replacement agents (wasted compute/tokens)
• Teammate can't report completion → team lead must poll external systems (git, APIs) instead of receiving status updates
• Breaks the coordination loop — team lead can assign work but can't receive results
• Error reporting impossible — teammate hit a 401 API error and tried to report it via SendMessage, message never arrived

Environment

• Claude Code CLI (latest, April 2026)
• macOS
• Background agent spawned via Agent tool with run_in_background: true
• Team created implicitly via team_name parameter

Related Issues

• #25135 — SendMessage silently succeeds when recipient name doesn't match inbox polling target
• #34668 — Teammates intermittently stop receiving SendMessage after extended polling
• #23415 — Teammates don't poll inbox, messages never delivered

This appears to be a distinct variant: the directionality is the key difference. Lead→teammate works perfectly, teammate→lead is completely broken.

Workaround

None. Team lead must poll git repos / external trackers for teammate progress instead of relying on team messaging.