[RFC]: Add coordination/unit-context-sync extension

[makotohonda1]

Summary

Add an opt-in Unit Context Sync (UCS) extension under a new coordination/ category that keeps units of work aligned during the Construction phase after a system is split into multiple units built in parallel. The model surfaces where a human-to-human discussion is needed before a unit-boundary change spreads, records decisions with their rationale, propagates human-made decisions across units at decision time, and reconciles consuming units against prior decisions.

Motivation

AI-DLC decomposes a system into units of work for parallel, independently deployable development. In practice, when units are built in separate sessions or by different people, their contexts diverge and drift from the assumptions agreed during Inception. That drift surfaces all at once at integration time and is expensive to reconcile — to the point that, for small systems, splitting can feel slower than not splitting at all.

The current workflow has no Construction-phase mechanism to (a) surface "this change needs a human-to-human discussion" before it spreads, or (b) carry a decision and its rationale across units at the moment it is made. audit.md is a chronological interaction log, not a structured, cross-unit-consumable decision record; parallel-team-review operates at Inception, not during Construction execution where the drift accumulates.

Detailed Proposal

A new opt-in extension at extensions/coordination/unit-context-sync/ with rules UCS-00..04 (blocking by default; marked N/A for single-unit projects):

• UCS-00 — Declare the cross-unit sync method (git / shared-fs / manual / other) at Construction start. The model asks; it does not choose.
• UCS-01 — Record unit-boundary changes/decisions with intent: the decision, its reason, the rejected alternatives, and a confidence/temperature (settled/contested/provisional), attributed to a human. Complements (does not replace) audit.md.
• UCS-02 — Surface cross-unit changes for human discussion: if a change still has room for disagreement, name the affected units and hold propagation until humans decide; never self-downgrade to "no discussion"; default to discussion when uncertain. At stage completion this is a blocking Coordination Finding — the stage offers only "Request Changes", not "Continue".
• UCS-03 — Propagate decided changes across affected units at decision time per the declared sync mode, with a per-unit note — not deferred to merge.
• UCS-04 — Reconcile against prior decisions before acting: refresh the coordination records, follow existing decisions by ID rather than re-deciding, and reconcile with a changed contract before proceeding.

Records live under a new aidlc-docs/coordination/ (sync-mode.md, contracts/, decisions/ledger.md + records/). The extension is sync-method-agnostic (the sync infrastructure is the team's operational choice and is out of scope). An opt-in prompt is presented during Requirements Analysis (recommended when units are built in parallel by more than one person or session).

Alternatives Considered

• Rely on the existing parallel-team-review: it reconciles team understanding at Inception, not during Construction execution where drift actually accumulates.
• Static contract tests / integration tests: these catch drift at integration time — the cost UCS is designed to avoid — and do not carry decision intent or trigger a human discussion.
• Use audit.md only: it is a chronological log, not structured decision intent, and is not designed for cross-unit consumption/reuse by ID.

Drawbacks

• Adds blocking gating during Construction; the value is greatest when two or more units are built in parallel. Single-unit/sequential projects mark the rules N/A or opt out, so there is no overhead there.
• To stay infrastructure-agnostic, UCS-03 makes the propagation timing explicit and prompts the action, but relies on the team's chosen sync method to actually deliver the records (it does not transport bytes itself).

Additional Context

Tested on Claude Code with a full greenfield AI-DLC run plus a separate naive-loader check; a reproducible test plan and results accompany the PR. Files: unit-context-sync.md (UCS-00..04, with per-rule Verification), unit-context-sync.opt-in.md, and a README "Built-in Extensions" entry. The PR diff is focused (2 new files + 4 README lines); sync infrastructure and any enforcement hooks are intentionally out of scope.