Add prospect-sequence runx skill

[zdfgu113]

Summary

• Add prospect-sequence, a runx skill for source-backed account research and outreach sequence drafting.
• Emit a gated send_proposal for send-as; the skill does not send messages or mutate external systems.
• Reject loopback/private/local/non-HTTP/off-allowlist sources before using account facts.

Verification

• runx-cli 0.6.13
• runx harness ./skills/prospect-sequence -R /home/zjs/runx-prospect-pr-check --json passed 2 cases.
• Published package: zdfgu113/prospect-sequence@sha-b4a3b8668802
• Public URL: https://runx.ai/x/zdfgu113/prospect-sequence
• Published package install, dogfood run, and receipt verification are captured under skills/prospect-sequence/references/.