Fix staleness comments to suppress owner pings when tooManyOwners

[Copilot]

makeStaleness() was unconditionally passing the full owner list to comment generators, causing staleness comments to ping 50+ owners when the tooManyOwners flag was set.

Changes

• Modified getStaleness() to check tooManyOwners before populating ownersToPing
• When true, passes placeholder ["«maintainers»"] instead of the full owner list
• Placeholder renders as @«maintainers» in comments—clear but non-pinging

This follows the existing pattern for the "no owners" case, which uses ["«anyone?»"].

Example

Before (with 60 owners):

Re-ping @user1, @user2, @user3, ... @user60:

After:

Re-ping @«maintainers»:

Original prompt

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This section details on the original issue you should resolve

<issue_title>mergebot staleness comments do not respect tooManyOwners</issue_title>
<issue_description>makeStaleness() unconditionally slings the owner list to the comment generator – it should be suppressing this output if too many owners.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes mergebot staleness comments do not respect tooManyOwners #1218

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