Retire embedded filesystem METHOD tooling after GitHub Issues migration

[flyingrobots]

Problem

Echo has migrated live Method backlog tracking to GitHub Issues, but the embedded METHOD tooling still assumes the old filesystem backlog exists.

Concrete evidence from the migration:

• crates/method/src/workspace.rs still discovers a workspace by requiring docs/method/backlog.
• PR Migrate Echo Method backlog to GitHub Issues #527 had to add docs/method/backlog/.gitkeep purely as a compatibility marker so fresh clones can still run legacy cargo xtask method ... commands.
• Legacy commands such as cargo xtask method inbox, matrix, dag, frontier, critical-path, and check-dag still describe or operate on the old filesystem backlog model.
• cargo xtask method status now reports zero legacy backlog files after migration, which is expected but misleading if treated as the live tracker.

This is bad code because the implementation encodes a retired workflow as a structural workspace invariant. The marker file is acceptable as a short-term bridge, not as architecture.

Desired outcome

Echo either removes the embedded filesystem-backlog METHOD command surface or replaces it with GitHub-Issues-aware behavior.

Acceptance criteria

• Fresh clones do not require a fake/empty backlog directory solely to satisfy Method workspace discovery.
• cargo xtask method status does not present the empty legacy filesystem backlog as the live tracker.
• Legacy filesystem commands are removed, explicitly deprecated, or redirected to GitHub Issues guidance.
• Docs and command help agree that GitHub Issues are the live tracker.
• Existing design/retro/legend docs remain discoverable without resurrecting live docs/method/backlog/** cards.

References

• Echo PR Migrate Echo Method backlog to GitHub Issues #527
• Merge commit c26e3ed505db0b8de4057dc53be2d94426486cf7
• Compatibility marker added in docs/method/backlog/.gitkeep

[coderabbitai]

🔗 Related PRs

#184 - Docs: align roadmap with milestone tracks (closes #183) [merged]
#309 - feat: add Doghouse PR flight recorder [merged]
#373 - chore: remove legacy ttd-browser surface [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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