docs: simplify README with MCP install and signup first

[escott-]

Summary

Align the README with the new, minimal MCP-first homepage. This is a README-only change. No runtime, installer, release, account, or security-policy behavior is changed.

• Lead with the small existing logo, "Persistent memory. Millisecond code search.", and "Your agent knows code. Give it the decisions behind yours."
• Put Start free with MCP, the 10,000-credit offer, compatibility, and account creation/sign-in explanation immediately before the install commands. The macOS/Linux and Windows commands appear on lines 28 and 34, before benchmarks or technical detail.
• Match the install commands shown in the maintainer-approved homepage: /mcp/install.sh | sh and /mcp/install.ps1 | iex.
• Make website signup secondary. Account onboarding is represented as part of MCP, consistent with the account work shipped in v1.0.7.
• Replace the long feature lists, benchmark comparison tables, badges, and repeated editor inventory with a first-use prompt and compact proof/data-control section.
• Preserve the hosted-indexing disclosure, enabled-by-default transcript/Git capture information, links to controls, and the distinction between the MIT client/server repository and the separate hosted backend.
• Keep npm, hosted MCP, dry-run, and diagnostics in a collapsible section. Link to setup, architecture, releases, contribution/security guidance, pricing, integrations, and benchmark methodology.
• Retain the mcp-name registry comment and legacy #install-in-30-seconds anchor. Close with "Let humans be human." No em dashes or new research-enforcement claims.

Validation

• Parsed the Markdown locally with HTML support and confirmed all six fenced examples render as code, the details block is balanced, and explicit IDs are unique.
• 19 local content/structure checks passed, covering command placement, secondary signup hierarchy, metadata/anchor preservation, existing repository link targets, disclosure text, and whitespace.
• The uploaded blob is byte-identical to the checked candidate: affe913af2ae0622df48e0f34626c1dec4e3e5e2.
• Reviewed the current README, data-handling document, contribution guidance, main-branch v1.0.7 account changes, and published benchmark page.
• No installer was executed and no account was created. This does not establish end-to-end onboarding success. Full Rust/application tests were not run locally for this prose-only change.

Before merge

• Smoke-test both homepage install aliases and integrated signup on the supported platforms. Live endpoint checks were unavailable from this editing environment; the commands match the supplied homepage, while the documentation crawler still shows older /scripts/... URLs.
• Maintainer review and DCO sign-off as required by CONTRIBUTING.md.

This PR does not modify the repository About description or PR #581 in the separate ContextStream repository.