docs: make the README a user-facing page

[Rinat S (rsolmano)]

Problem

The README opened with a quarter-page of signing, notarization, and packaging caveats before the first
install command, then spent half its length on contributor material. A new visitor had to read past all of
that to find curl ... | bash.

This supersedes #514 by Iaroslav (Rick) Postovalov (@CommanderTvis). His commit keeps its authorship and message; rebasing it onto
main resolved the intro conflict with #643. A separate follow-up commit polishes the result and
addresses the review findings.

Approach

Split the two audiences. The README leads with the pitch and the install commands, keeping only what a
downloader acts on: requirements, per-platform install steps, installer options, platform/signing support,
and the privacy choices. Contributor material moves to CONTRIBUTING.md.

Notable decisions:

• docs: reposition ThinkRail as the agentic IDE that gets better with use #643 positioning kept verbatim; the worktree paragraph now directly follows the pitch.
• Website link uses ?utm_source=github&utm_medium=readme rather than docs: make the README a user-facing page #514's jb.gg/osk2n7, which
  resolves to utm_source=blog and would count README visits as blog traffic.
• Installer options stay in the README, with a copyable bash -s -- form and X.Y.Z placeholders
  instead of concrete versions that go stale. The website's install section links to #install for
  "advanced CLI options" and "native library requirements", and the intro blog post links there for the
  CLI's install, channel, and update options.
• Privacy keeps every user-decision fact while staying compact: basic events are always on and linked
  by an installation ID; optional sharing is on by default and starts when the first-launch dialog opens;
  --no-analytics is per-run; the one-time blog open and its marketing-cookie condition.
• CONTRIBUTING links the package map in architecture.md instead of copying a repo tree that had
  already drifted from the packages on disk.

Changes

• README: pitch → Install (desktop app, command line, platform support) → Analytics & privacy →
  Contributing → License. 250 → 139 lines.
  • Desktop steps per platform, verified against the v0.1.5 and nightly.21 assets: DMG with an
    Applications link; Windows ZIP whose setup executable (ThinkRail-Setup.exe / -canary.exe) needs its
    adjacent .installer payload; Linux tarball with ./installer.
  • Platform table verified: the published DMG passes stapler validate / spctl as Notarized Developer
    ID; the macOS CLI is Developer ID signed.
• CONTRIBUTING: gains the launcher commands, "Artifact signing" (updated — published DMGs are notarized now;
  local packages are unsigned), a short "Architecture (three rings)", e2e:binary / e2e:desktop, and the
  note that thinkrail update from a checkout installs a published binary.

Verification for this docs-only change: bun run lint → clean; git diff --check origin/main..HEAD → clean;
every relative link and the architecture.md#topology--three-rings anchor resolve.

Screenshots

Not applicable — no frontend changes.

Related issues

Supersedes #514.

Checklist

• Fast gates pass: bun run lint, bun run typecheck, bun run test — only bun run lint ran (clean); typecheck and unit tests were not run because no code changed
• E2E suite passes for app-affecting changes (bun run e2e, or bun run e2e:full when touching agent behavior) — not applicable, no app behavior changed
• Before/after screenshots are included for frontend changes, or marked not applicable
• Relevant SPEC.md / top-level specs updated to reflect any boundary, contract, or behavior change — not applicable, no boundary/contract/behavior change
• I have read the Contributing guide and agree to the Code of Conduct

[jetbrains-air]

Approved — ready to merge.

[Iaroslav (Rick) Postovalov (CommanderTvis)]

I am a bit confused about why this is closed.