Gmail and Calendar are handled together because both Google connectors share
the _build_credentials helper that lived in shared/tools/gmail.
- relocate the gmail helpers (_get_token_encryption, _build_credentials,
_gmail_headers, _format_gmail_summary) into the gmail subagent slice
(tools/_helpers.py); repoint gmail search_emails/read_email to it.
- calendar search_events now imports _build_credentials from the gmail slice
(preserving the existing cross-connector Google-auth dependency).
- repoint both dead tools/__init__ shims at the live local impls.
- fix tests/e2e native_google fake: it patched the dead shared
google_calendar.*.build paths; point it at the live subagent calendar
modules (which actually import googleapiclient build).
- delete dead shared/tools/{gmail,google_calendar} twins.
shared/tools now has zero connector dirs. agents unit suite green (942).
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