docs: improve README with badges, intro, and em dash fixes

[imran-siddique]

Summary

• Adds three badges (license, TRACE spec link, conformance test count)
• Adds a brief implementer-facing intro explaining why you would run this suite
• Removes all em dashes (replaces with hyphens or rephrases)
• Clarifies the Status section: spec publishes at CC Summit June 23, test suite usable then, certification program is a separate 2027 timeline

Test plan

• README renders correctly on GitHub (badges resolve, tables display, no em dashes visible)
• Badge links go to the right targets (LICENSE file, agentrust-io/trace-spec repo)

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