feat(web-fetch): add OmniRoute web fetch provider

[ekinnee]

Summary

• add an OmniRoute-backed OpenClaw web-fetch provider for POST /v1/web/fetch
• map the OpenClaw 2026.7.1 contract (url, extractMode, and host-owned maxChars) without inventing provider-specific fallback behavior
• preserve standard content, final URL, title, extractor, and upstream truncation fields while keeping response and error reads bounded
• update manifest, package metadata, README, support roadmap, registration tests, and packed-artifact compatibility coverage

Closes #16

Contract and safety notes

OmniRoute is always requested in markdown because its endpoint does not accept OpenClaw's plain-text mode. Plain text is projected with OpenClaw's public markdownToText helper. OpenClaw owns final maxChars normalization; the adapter does not truncate before host normalization. Private, loopback, and other special-use target hostnames are rejected before forwarding. Provider selection, depth, selector wait, links, descriptions, and screenshots remain deferred because they are not representable in the current public OpenClaw web-fetch contract.

No provider fallback is added: OmniRoute retains ownership of its configured web-fetch routing and quota policy.

Validation

• pnpm test — 150 tests passed
• pnpm build
• git diff --exit-code -- dist/
• git diff --check
• node .github/scripts/upstream-compatibility-probe.mjs --source-contract
• packed artifact installed with OpenClaw 2026.7.1; all nine registrations passed exactly once, including web fetch, with envVars limited to OMNIROUTE_API_KEY
• one bounded direct OmniRoute probe reached 192.168.1.254:20128; the gateway returned its explicit 400 that no web-fetch provider credentials are configured, so no live success is claimed

ClawSweeper is not installed in this checkout, so that local gate was unavailable.