docs: enhance README (badges, positioning)

[stukenov]

OSS marketing pass on the README — enhances the existing content without rewriting or removing anything.

Changes

• Positioning line: added a crisp one-liner framing PostPilot as a self-hosted, terminal-first alternative to Buffer/Hootsuite (no SaaS subscription, no vendor lock-in).
• Telegram backend fix: the Supported Platforms table claimed Telegram used Playwright, but scripts/telegram_publisher.py publishes via the Telegram Bot API (api.telegram.org/.../sendMessage). Corrected to reflect the real implementation.
• Install fix: replaced the YOUR_USERNAME placeholder in the clone command with the real stukenov/postpilot URL.
• Related projects: added a section linking keste.

Notes

• Badges: license and Python badges already present and kept. PyPI badge intentionally not added — the postpilot package is not published on PyPI (pypi.org returns 404), so a version badge would be misleading.
• Quickstart verified against the real CLI (cli.py + scripts/*_publisher.py): export-txt, build-queue, run, whoami, list-posts, post-insights, user-insights all exist as documented — no changes needed there.
• The existing "Why PostPilot?" comparison table already covers hosted schedulers, so no duplicate comparison note was added.
• No substantial content removed; all edits are additive or factual corrections.

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