Migrate Echo Method backlog to GitHub Issues

[flyingrobots]

Summary

Migrate Echo filesystem Method backlog cards into GitHub Issues and archive the source cards so GitHub Issues become the live tracker.

Scope

• Convert docs/method/backlog/** cards into GitHub Issues with source-file provenance.
• Preserve migrated cards under the graveyard as historical evidence.
• Record the migration mapping in repo docs.
• Leave active work in GitHub Issues, not filesystem backlog lanes.

Acceptance criteria

• Method lane/legend/type/priority labels exist in flyingrobots/echo.
• Backlog cards are represented as GitHub Issues.
• Original source cards are archived out of live backlog lanes.
• Archive README records issue-to-source mapping.
• Branch is committed and pushed for review.

[coderabbitai]

🔗 Related PRs

#92 - chore: GitHub templates (bug/feature/task/PR/RFC) [merged]
#104 - docs(AGENTS): PR↔Issue linkage policy [merged]
#184 - Docs: align roadmap with milestone tracks (closes #183) [merged]
#385 - docs(backlog): two follow-ups from the 2026-05-30 merge session [merged]
#388 - docs(method): add backlog cards and hook failure note [merged]

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