assign_to_agent: allow multiple assignments to same issue when pull_request_repo differs

[camposbrunocampos]

Summary

When assign_to_agent is called twice for the same issue but with different pull_request_repo values (e.g. a multi-platform request targeting
Husband for iOS and Husband-Droid for Android), the second assignment is silently skipped.

Root Cause

In assign_to_agent.cjs, the already-assigned guard short-circuits as soon as Copilot appears in currentAssignees, regardless of whether a different
pull_request_repo was requested:

if (currentAssignees.some(a => a.id === agentId)) {
  core.info(`${agentName} is already assigned to ${type} #${number}`);
  results.push({ ..., success: true });
  continue;  // ← second assignment is never made
}

Observed Behavior

Successfully assigned copilot coding agent to issue #6587
Waiting 10 seconds before processing next agent assignment...
Using per-item pull request repository: perrystreetsoftware/Husband-Droid
copilot is already assigned to issue #6587   ← second assignment skipped

The second call had a different pull_request_repo and should have triggered a new Copilot session in that repo.

Proposed Fix

Only skip when no explicit pull_request_repo is provided:

if (currentAssignees.some(a => a.id === agentId) && !effectivePullRequestRepoId) {
  core.info(`${agentName} is already assigned to ${type} #${number}`);
  results.push({ ..., success: true });
  continue;
}

This preserves the no-op dedup behavior for plain re-assignments while allowing Copilot to be triggered for a second platform (different 
pull_request_repo) on the same issue.

[pelikhan]

Treat the pairs of assignee/target repo as the uniqueness key used to avoid duplicating an assignment

[pelikhan]

@copilot skip duplicate detection when a different repo is specified.