fix(desktop): reuse team agent identities

[dpwai-admins]

Summary

• Reuse a persona-backed managed agent when it is already attached to the target channel.
• Refresh the batch reuse context after every successful attach so repeated personas cannot mint twice in one deployment.
• Keep explicit forceNewInstance requests working.
• Remove the unconditional fresh-instance flag from Deploy team to channel.

Closes #3639.

This is a focused replacement for the desktop portion of stale/conflicting #3646. It also preserves the current access-policy reconciliation when an existing agent is reused.

Validation

• 44 focused agent reuse/team deployment tests passed.
• TypeScript typecheck passed.
• Desktop lint and static checks passed.
• Full Desktop suite on commit 28e74bb: 6,454/6,455 passed.
• The sole failing useKnownAgentPubkeys timing assertion also fails unchanged when run alone on clean main at 3c7f288 (same expected/actual ordering), so it is pre-existing and outside this patch.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 3c7f288c60d67df78577b237e27c3dfc8831aaa1...28e74bbf3a24ba9e58b3662a8f4793f6f362e3b0.
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