docs: rewrite the README as a landing page

[starvy]

The README read as a manual rather than a pitch. 186 lines / 1273 words, of which Secrets
alone was 62 lines and 489 words — 38% of the document, more than Why + Status + Install
combined. A backend-selection matrix containing a D-Bus error string sat where every visitor had
to scroll past it.

Now 127 lines / 624 words.

How the target was chosen

I benchmarked the actual READMEs of comparable projects rather than guessing. Two archetypes:

Archetype	Examples	Words
README is the manual (CLI, no docs site)	bat, ripgrep, starship	1,395–4,654
README is a landing page (GUI app + docs/)	yaak, zed, posting, helix, git-cliff	195–494

Posel has a GUI and a docs/ directory, so it's the second kind — and was sitting at 2.6× yaak
(18.9k stars, 494 words) and 1.5× bruno.

Changes

• One claim, not four. Tagline is now "A native API client for REST, GraphQL, and tRPC. Your
  requests are files." followed by three standalone sentences ending on an absolute — "No account,
  no sync server, no telemetry. Not now, not later." Bruno's equivalent "ever" is the most-quoted
  word in that repo.
• Shows the file format inline. A 13-line toml block that is byte-identical to
  examples/github-api/User/get-a-user.toml, with {{variables}} and [[assertions]] visible.
  "Your requests are files" is abstract; the snippet isn't. No comparable project does this — it's
  the ripgrep-benchmark-table move: prove the claim instead of asserting it.
• Surfaces what was undersold. tRPC (the GitHub repo description already advertised it, the
  README didn't — a visitor reading both saw a contradiction), assertions + posel-cli test,
  OpenAPI/curl import, codegen. Bruno gives its CLI a whole section; yaak has no CLI at all.
• Adds "Why shouldn't I use Posel?" — pre-1.0, unsigned builds, no Postman-collection import.
  Stolen from ripgrep. Preempts the top comment on any launch thread.
• Four badges (CI, release, license, Rust), one line. Deliberately no star or download counts.
• Moves detail out: Secrets → docs/SECRETS.md (6 lines remain in the README), crate table →
  CONTRIBUTING.md, build steps and the gpui-pin rationale → CONTRIBUTING.md.

Adds CONTRIBUTING.md, which the repo didn't have — the cuts needed somewhere to land, and
every high-star repo in the sample links to one.

Verified

• Every relative link resolves (checked, not eyeballed)
• The quoted toml is diff-identical to the real example file
• posel-cli send / test invocations run as documented
• Counts corrected against the repo: 36 requests, 7 collections, 2 environments (README previously
  said "~30")

One thing to fix before this lands publicly

The v0.1.0 release is still a draft, so both the Release badge and the Download link point at an
empty Releases page. Publishing the draft (or cutting a new tag) makes the README's primary CTA
work. Everything else here is independent of that.