fix: scope per-unit artifact loading to unit directories in session-continuity

[Mameta29]

Summary

Closes #275.

The "Per-Unit Design" artifact-loading instruction in session-continuity.md did not reflect the actual per-unit directory structure of the Construction phase. A resumed, zero-context session had no way to know which unit's artifacts to load or where they live.

Problem

session-continuity.md line 33 read:

- **Per-Unit Design**: Read functional-design.md, nfr-requirements.md, nfr-design.md, infrastructure-design.md

This is inaccurate in three ways (verified against the current rule files):

1. Missing the {unit-name} directory level. Every Construction stage writes to aidlc-docs/construction/{unit-name}/{stage}/ (e.g. functional-design.md lines 68-71). The line listed bare filenames with no path.
2. No unit selection for multi-unit projects. Construction runs a per-unit loop; an N-unit project has N copies of these directories. The line gave no guidance on which unit(s) to load.
3. Filenames did not match the real artifacts. There is no single functional-design.md — the stage produces functional-design/business-logic-model.md, business-rules.md, domain-entities.md; similarly for nfr-design and nfr-requirements.

The other lines in the same list are fine because those stages are not unit-scoped — only Per-Unit Design has artifacts under a {unit-name} directory.

Change

Rewrites the Per-Unit Design line to:

• Use the real path shape aidlc-docs/construction/{unit-name}/{functional-design,nfr-requirements,nfr-design,infrastructure-design}/.
• State unit selection explicitly: determine the in-progress unit from aidlc-state.md, load that unit plus its dependencies (per unit-of-work-dependency.md).
• Defer exact filenames to the construction stage rules (single source of truth) instead of repeating a stale, inaccurate list.

Test Plan

• Observed during a multi-unit (6-unit) greenfield Construction run: on a mid-Construction resume, the instruction pointed at filenames that do not exist and are not unit-qualified.
• The changed file passes markdownlint-cli2 (0 errors).

Checklist

• Reviewed the contributing guidelines
• Performed a self-review
• Changes tested (multi-unit Construction run described above)
• Changes documented ([Bug]: session-continuity.md Per-Unit Design loading is not unit-scoped — resumed sessions cannot locate per-unit artifacts in multi-unit projects #275 has full context)

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.

[harmjeff]

LGTM! Clean, well-scoped fix for a real bug that hits multi-unit Construction projects on session resume.

Good approach — referencing the construction stage rules as the source of truth for exact filenames rather than duplicating a stale list here. The unit selection logic (check aidlc-state.md + load dependencies per unit-of-work-dependency.md) is correct and actionable for an agent resuming with zero context.