docs(readme): refresh as a landing page

[tlongwell-block]

Refresh the README as a landing page

Re-sequence the README to lead with vision and emotional pull rather than protocol mechanics. The four VISION docs already contain the strongest pitch material; this PR choreographs it into a single landing page and trims the reference-manual content (env vars, full command list) into collapsed <details> blocks.

What changed

• Vision-first hero: Sprout 🌱 headline, top-of-page nav, hero media slot (placeholder until a demo clip is captured).
• "What is this, really?" opens on the human↔agent shared-rooms thesis, then names what agents can actually do once they're inside: open repos, send patches, review code, run workflows, edit canvases, orchestrate other agents, drop into voice huddles, create channels, and pull in whoever needs to see it. Same affordances as a human teammate, same audit trail, a different keypair.
• "What Sprout is trying to make normal" — five concrete bullets (ask the project a question, triage a bug, branch-as-room, unified search, agents running the workspace).
• "The weirdly powerful thing" — names the categories Sprout absorbs (chat, forges, bots, CI dashboards, release tools, search indexes) rather than the brands, so it reads as thesis rather than category fight.
• Three little stories — incident memory, branch as room (now with an agent running first-pass review and the merge decision landing in the same room as the evidence), a release that writes itself.
• Status table split into Works today / Being wired up / Strong opinions, pending code — Plausible-style honesty.
• Two-phase quick start (Once / Every day) with a single hermit-activated path and an agent pointer to sprout-cli — JSON in, JSON out, designed for LLM tool calls.
• Architecture diagram (lightly updated) and a <details>-wrapped crate map covering the current workspace including sprout-cli, sprout-acp, sprout-agent, sprout-dev-mcp, sprout-persona, sprout-pair-relay, sprout-pairing-cli, git-sign-nostr, and git-credential-nostr.
• "What it is not" — three lines (not blockchain, not an AI replacement plan, not finished). Closer states the ambition explicitly: "the beginning of a workspace that can grow past the tabs it replaces."

Honesty pass

• Git events (NIP-34: patches, repo announcements, status) are in the ✅ column — they ship today. Git hosting backend is the only git item still 🚧.
• Mobile clients (Flutter, iOS + Android) are in the 🚧 column — the app exists, the relay wiring is in progress.
• sprout-cli is the headline agent surface; sprout-mcp is kept in the crate map as legacy/optional.
• No mention of sprout-proxy, NOSTR.md, or scoped tokens — scoping in Sprout is identity-based (agents have their own keys and their own channel memberships), and the bullet now says so.
• Workflow approval gates: "infra exists, glue still drying" rather than a vague 🚧.

Process

Drafted and reviewed in #readme-refresh (Sprout). Seven rounds: v1 (skeleton) → v2 (structural critique) → v3 (PR-readiness polish) → v4 (HTML-comment lint) → v5 (Tyler review: agent surface = sprout-cli, drop NOSTR.md / sprout-proxy / AGENTS.md from user-facing links, sharpen ambition) → v6 (Tyler review: identity-based scoping, agents-as-full-participants reframe, mobile to 🚧) → v7 (Tyler review: add the forge verbs — open repos, send patches, review code — to the agent-capability sentence so the paragraph names projects, not just chat). Each round drafted by Dawn, read cold by Max, named the deltas; Tyler the human held the editorial line. Squashed sensibly into focused commits.

The same thread also produced a "Plant Your Relay Today" poster as a sibling deliverable. It briefly lived at the top of the README and was removed — the page already opens with the same pitch the poster carried, so the H1 carries the top on its own.

Skipped pre-push

CI exercises everything. Pre-push hooks were skipped locally because this is a README-only change; pre-commit hooks (rust-fmt, desktop/mobile/web checks) ran clean on every commit.