Add DCO, trademark policy, update ownership to AgentTel Ltd, migrate repo URLs

[rrohitramsen]

Summary

Prepares the repo for transfer from rrohitramsen/AgentTel to agenttel/agenttel-sdk.

• Add standard Developer Certificate of Origin (v1.1) file
• Add TRADEMARK.md naming AgentTel Ltd (Company No. 17098920) as the trademark owner
• Update CONTRIBUTING.md with DCO sign-off requirements (git commit -s)
• Update LICENSE copyright from "AgentTel Contributors" to "AgentTel Ltd"
• Update all repo URLs across 13 files from rrohitramsen/AgentTel to agenttel/agenttel-sdk
• Update docs site URL to agenttel.github.io/agenttel-sdk
• Update package.json author to "AgentTel Ltd"

Files changed

File	Change
DCO	New — Developer Certificate of Origin v1.1
TRADEMARK.md	New — trademark usage policy referencing AgentTel Ltd
LICENSE	Copyright → AgentTel Ltd
CONTRIBUTING.md	DCO section + repo URLs
README.md	Repo + docs URLs
build.gradle.kts	POM SCM URLs
gradle.properties	POM URLs
mkdocs.yml	Site + repo URLs
agenttel-web/package.json	Author + repo URL
src/javadoc/overview.html	Repo URL
docs/index.md	GitHub link
docs/getting-started/quick-start.md	Example links
docs/project/contributing.md	Clone URL + issue links
CHANGELOG.md	Release tag URLs

Post-merge steps

1. Transfer repo via GitHub Settings → Transfer to agenttel org, rename to agenttel-sdk
2. Enable DCO bot on the org
3. File UKIPO trademark application for "AgentTel"

Test plan

• Verify no remaining rrohitramsen/AgentTel references (except personal developer identity in POM)
• Verify LICENSE says "AgentTel Ltd"
• Verify TRADEMARK.md references company number
• Verify ./gradlew clean build still passes