Add Instagram posting strategy

[Copilot]

Adds an InstagramStrategy for crossposting to Instagram. Unlike other strategies, Instagram requires an image, so post() rejects when none is provided.

Changes

• src/strategies/instagram.js — New InstagramStrategy (accessToken + accountId). Uses the Instagram Graph API content-publishing flow: create media container → publish → fetch permalink. Uses the first image as media and the message as caption. MAX_MESSAGE_LENGTH = 2200; getUrlFromResponse() returns the permalink.
• src/index.ts — Exports the strategy and its types.
• src/bin.js — -i/--instagram CLI flag; reads INSTAGRAM_ACCESS_TOKEN and INSTAGRAM_ACCOUNT_ID.
• README.md — API/CLI usage, env vars, and setup instructions.
• tests/strategies/instagram.test.js — Covers constructor validation, image-required behavior, publish flow, API/abort failures, URL extraction, and length calculation.

const instagram = new InstagramStrategy({
	accessToken: "your-access-token",
	accountId: "your-instagram-account-id",
});

// Throws TypeError: "Instagram requires an image to post."
await instagram.post("Hello world!");

await instagram.post("Hello world!", {
	images: [{ data: imageData, alt: "Description" }],
});

Note for reviewers

The Graph API officially publishes images via a public image_url, not binary upload. To stay consistent with this library's Uint8Array image convention (used by all other strategies), the image is sent as multipart form-data. Open to switching to a hosted-URL input if preferred.