chore: release 3.0.1

[vvillait88]

Summary

Releases 3.0.1, carrying everything merged since 3.0.0:

• the quota denial message no longer tells an agent to retry; both quota deny sites in core.ts now say verification is unavailable for this merchant and that retrying will not help (fix: quota denial message no longer tells the agent to retry #144)
• A2A signing and transport docs and supported versions corrected (docs: correct A2A signing and transport docs, supported versions; drop em-dashes #143)
• README: the UCP example imports the payment-handler builders it uses (docs: import the payment-handler builders the UCP example uses #145), and UCP profile keys[] is informational (README: UCP profile keys[] is informational #142)

Also in this PR: @agent-score/sdk floor raised to 2.8.1, released today.

Worked with: Varun.

Out of scope: nothing else is pending on main.

Type of change

• Bug fix (no breaking change)
• New feature (no breaking change)
• Breaking change (existing callers must update)
• Docs, tests, or internal maintenance only

Public API

none. The denial reason code and response shape are unchanged; only the message text differs.

Test plan

bun run lint, typecheck, test (1849 passed, 4 skipped) and build pass locally on this branch.

Checklist

• Tests cover the new behavior, and the suite passes locally
• Lint, format, and type checks pass
• Docs and README examples updated if the public surface changed
• No secrets, credentials, or personal data in the diff or the tests