TT-4413: add agent account, policy, and rule management

[qasim-nylas]

Summary

• move managed agent grant operations under nylas agent account, add account get, and support --policy-id during agent account creation
• add agent-scoped nylas agent policy CRUD with default-account scoping, --all visibility across provider=nylas accounts, richer text rendering, and attachment guardrails
• add agent-scoped nylas agent rule CRUD with default-policy scoping, common-case flag-based create/update flows, scope validation, and rule/policy safety checks
• add detailed command docs for agent policy and rule workflows, and expand unit/integration coverage for the new account/policy/rule paths

What Changed

• renamed the original flat agent account commands into the nylas agent account namespace and kept nylas agent status as the top-level health/check entrypoint
• added new policy domain/port/adapter implementations plus demo/mock clients for policy CRUD
• added new rule domain/port/adapter implementations plus demo/mock clients for rule CRUD
• implemented nylas agent policy list|get|read|create|update|delete
• implemented nylas agent rule list|get|read|create|update|delete
• made agent policy list and agent rule list resolve through provider=nylas accounts and attached policies instead of exposing app-global resources directly
• added rule creation/update flag UX for common cases: --name, --description, --priority, --enabled / --disabled, --trigger, repeatable --condition, repeatable --action
• added guardrails to block deleting attached policies, deleting the last rule on an attached policy, and mutating rules that fall outside agent scope or are shared outside agent scope
• split out dedicated docs for policy and rule usage under docs/commands/agent-policy.md and docs/commands/agent-rule.md
• added supporting reliability/test fixes in the Nylas adapter and integration suite used by this branch

Validation

• make test-integration
• make test-air-integration

[AaronDDM]

LGTM 👍

[AaronDDM]

LGTM 👍