docs: rewrite root vision and agents policy for current surface

VISION.md was written Feb 21, 2026, before source-native
continuity, the dreaming loop, and the local-substrate framing
existed. It also leaned on EIP-8004 wallet identity and a
federated memory-relevance model that are not on the product
roadmap. The new draft drops both, tightens to four-to-five
paragraphs, and lands a 'what Signet is not' product-positioning
list scoped to NOT duplicate the contribution-policy list in
AGENTS.md.

AGENTS.md is restructured to match the openclaw/AGENTS.md shape:
telegraph bullets, hard policy and routing only, no area
guidance, no teaching prose. Key additions: a 'What We Will Not
Merge' charter, a 'Proof' section patterned on openclaw/Validation
(live-verify, dependency-contract proof, no API/default guesses),
a 'Git / PR / Commit' section with hard rules, a sibling-harness
inspection gate for integrations/<harness>/ work, and a
sibling-harness directory note covering references/pi-mono/ and
the harness-name-mismatch reality.

Cut: philosophy prose, definition-of-done motivation, area
guidance blocks (deferred to scoped package AGENTS.md files in
follow-up PRs), the package map table (now a pointer to
docs/ARCHITECTURE.md), the inline build order (now a pointer to
root package.json scripts).

VISION: 100 -> 120 lines. AGENTS.md: 476 -> 366 lines.

Author: NicholaiVogel