epic: subtract zero-adopter tracker and compatibility machinery

[sungjunlee]

Outcome

Shrink tracker and compatibility machinery to the smallest justified surface after mirrorless GitHub operation is proven.

Scope

• Measure code, test, and documentation owned only by the zero-adopter local/non-default tracker axis.
• Decide whether the local adapter becomes a separately installed legacy skill/export tool or is removed.
• Keep Backlog.md compatibility one-way where useful; do not preserve bidirectional lifecycle complexity solely for format compatibility.
• Add absence tests proving the core cycle works with no Relay, Matt Pocock skills, craftkit, GitHub Project, or Backlog.md tooling.
• Remove dead compatibility contracts rather than replacing them with another general adapter framework.

Acceptance criteria

• Every retained tracker abstraction has a named current consumer or a measured portability invariant.
• Zero-adopter machinery is removed or isolated behind an explicit legacy boundary.
• GitHub remains the standalone default with no runtime fallback to another tracker.
• Backlog.md compatibility is documented as import/export/legacy behavior, not a second lifecycle authority.
• Optional integration absence is covered by tests.
• Net product surface and maintenance burden decrease; this epic does not add a new provider.

Dependencies

Starts after the mirror-off pilot has passed.

[coderabbitai]

🔗 Related PRs

#286 - feat: move GitHub behavior behind tracker adapter (#275) [merged]
#303 - feat: prove github and local tracker cycles end to end (#278) [merged]
#343 - remove the progress-sync and sprint-mirror axis (#340) [merged]
#351 - docs: define GitHub-native authority contract [merged]
#353 - feat: make GitHub workflow mirrorless by default [merged]

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[sungjunlee]

Implementation evidence: PR #354 removes the zero-adopter local tracker and generic/local design surface, with four exact whole-file deletions (1,909 lines) and a net product diff of 2,567 fewer lines at PR open. The retained GitHub seam is mapped to named consumers and executable portability invariants in docs/compatibility-subtraction.md. GitHub-only setup/no-fallback, legacy-config fail-closed and atomic publication, sprint-init/sprint-close/triage no-effect rejection, one-way Backlog.md legacy export, and absence of Relay/craftkit/Projects/Backlog.md tooling are covered by tests. Verification: focused 33/33, full Node suite exit 0, shell smoke 191/191, Linux CI pass, Windows CI pass, CodeRabbit pass, Codex P2 addressed and resolved, independent final review no findings.