feat(skill): add plan-first mode for manual edge routing

[sanpatricky]

Problem and value

When the skill encounters edge overlaps requiring manual routing, the current instructions lead to iterative trial-and-error: guess coordinates → validate → fail → adjust → repeat. In a real-world workflow diagram with 15 edges, fixing a single overlap took 4+ iterations because the skill added manual via routing when simply removing route: "drop" would have let port spread resolve it automatically.

Fixes #336

Scope

• What changed:
  • archify/SKILL.md step 3: added plan-first mode workflow (steps a–e) triggered when validation reports geometry errors or visual inspection reveals edge overlaps. Key addition: prefer automatic over manual — check whether removing a spread-disabling control resolves the overlap before resorting to manual via.
  • archify/SKILL.md line 46: added exception to docs-reading ban — when manual routing is needed, reading authoring-contract.md route-preset and port-spread sections is allowed before attempting fixes.
  • archify/references/authoring-contract.md § Spacing and labels: added batch-planning exception to the "one control at a time" rule — when multiple edges share a spatial channel, plan all coupled controls as a batch using --layout-json evidence.
• What deliberately did not change: renderer behavior, validator rules, schema fields, port spread logic, template HTML.
• No unrelated changes: confirmed.

Stability impact

• Compatibility and migration risk: None. Only skill instructions and authoring-contract documentation changed. Existing diagrams and typed JSON are unaffected.
• Renderer, validator, package, or generated-artifact risk: None. No code changes.
• Failure behavior and rollback path: Revert the two commits. No migration needed.

Tests run

Not applicable — documentation-only change. No renderer, validator, or package code modified.

Visual evidence

Not applicable — this changes skill authoring instructions only, not rendered output.

Generated artifacts

None. No renderer source, schema, or example input changed.

Checklist

• I used a minimal focused change and preserved existing typed JSON behavior unless the issue requires a contract change.
• I ran the relevant targeted tests and npm test in archify/. — N/A: documentation-only change
• I added or updated a regression test for behavioral changes. — N/A: no code change
• I checked generated artifacts and package freshness when their sources changed. — N/A: no source change
• I removed secrets, private repository content, and customer data from fixtures and screenshots.