docs: overhaul README around the ground-and-govern positioning

[saldestechnology]

Why

The docs site (docs.agentis.tools) and docs/messaging.md were repositioned around a single thesis — ctx is the local quality authority for AI-written code — but the README had fallen out of step. Its opening already carried the new positioning, yet the body was stale: organized as "Two Tools in One" (the old mental model), 830 lines of per-command flag dumps that now duplicate the docs site, and the killer workflow (ctx harness init wiring ctx into Claude Code) barely mentioned.

What changed

Rewrote README.md as a lean front door (~285 lines) that quickly conveys what ctx is and why to care, and stays accurate to the v0.3.1 CLI.

• Lede — pain + thesis, aligned verbatim with docs/messaging.md, unpacked into scannable Ground / Govern bullets with a concrete proof number (~8k tokens instead of 233k).
• The loop: index → ground → govern — one real terminal session; ctx index first; proof numbers woven in.
• One model, two jobs — replaces "Two Tools in One". Context generation (ctx src/ | pbcopy) folded in as one capability under Ground. The 0/1/2/3 exit-code table as the integration API.
• Drop it into Claude Code — the harness loop (SessionStart→map, PostToolUse→index+check, Stop→score) is now a headline, including the permission guardrails.
• Compact command table linking to docs, replacing ~25 expanded flag sections.
• Accuracy fixes — MCP flagged feature-gated; canonical check/score --fail-on vocabulary (no stale audit --min-score/--incremental showcase); every doc link verified against a real page.

Verification

• All commands, flags, and exit codes cross-checked against src/cli.rs / src/exit.rs.
• Taglines match docs/messaging.md (the source of truth).
• All 18 documentation links resolve to existing pages.
• ctx check passes clean (docs-only change).