feat(workflows): add flexible execution shapes

[The-Rabak]

Summary

• make vertical-slices the default execution shape while explicitly supporting infra-track and fix-batch when slices would be misleading
• add a shared execution-shape reference and thread it through plan, deepen-plan, work, execution prompts, inline review, tests, and generated platform outputs
• refresh README/changelog messaging and resync the OpenCode and OpenViking export surfaces to match the new workflow contract

Changes

• portable workflow prompts now declare and consume execution_shape, including the new shared reference at portable/compound-engineering/commands/workflows/references/execution-shape.md
• /workflows:work and its prompt templates now operate on generic execution units so non-slice modes remain first-class instead of going through fake slice adaptation
• generated Copilot and Claude surfaces were rebuilt, and the workflow contract tests were expanded to enforce the shared execution-shape reference

Testing

• bun run build:platforms
• bun run verify:generated
• bun test

Post-Deploy Monitoring & Validation

• What to monitor: no production systems; this is a workflow/docs/generated-output change
• Expected healthy behavior: generated outputs stay in sync and workflow commands expose the new execution-shape contract consistently
• Failure signals: stale generated outputs, broken command references, or contract test regressions
• Validation window: CI run for this PR
• If no operational impact: No additional operational monitoring required: repository-only workflow contract change.

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