Have I committed a sin by not informing my manager that they want to promote me above him? Is it a sin to demonstrate my competence, which would weaken his position? And do I have to deal with those who wish to harm me?

You are not sinful for not informing your manager of your intention to seek a promotion, for keeping matters secret helps in their accomplishment, as the Prophet, peace and blessings be upon him, said: "Seek help in achieving your needs by keeping them secret, for every possessor of a blessing is envied."

It is not permissible to strive to harm anyone, for the Prophet, peace and blessings be upon him, said: "There should be no harm nor reciprocal harm." And demonstrating competence by perfecting one's work is religiously required, for the Prophet, peace and blessings be upon him, said: "Indeed, Allah loves that when one of you does a work, he does it proficiently."

The Sharia recommends and rewards forgiveness and reconciliation, and it permits the oppressed to retaliate against those who wronged them without exceeding limits. Allah Almighty says: "And that you pardon is nearer to righteousness" and "So whoever pardons and reconciles, his reward is with Allah." You are advised to forgive those who sought to harm you, and in the future, guard against their harm and repel it with what does not involve injustice to them, and fear Allah in that.