refactor: split create() into preparePlan + execute

[ndeloof]

What this PR does, in one sentence

create()'s planning half — resolving the project, ensuring images/models and external resources, observing the daemon, and reconciling a Plan — becomes its own function, preparePlan, so a future caller can get the plan and snapshot without also running it.

Context

create() (pkg/compose/create.go) currently does two very different things in one unbroken call chain: it figures out what needs to happen (resolve the project model, ensure images/models exist, check external networks/volumes, observe the daemon, reconcile all of it into a Plan), then immediately does it (executePlan). That's fine as long as every caller wants both halves back to back — which is true today, but won't stay true once up converges onto the plan engine (epic #14081): interactive up needs to prepare the plan once, run its Create phase, set up the attach/printer/monitor session, and only then run the Start phase under a different context — it needs the plan and the daemon snapshot in hand between those two steps, not buried inside a single function that also executes everything.

What the PR brings

preparePlan(ctx, project, options) now returns exactly what a caller needs to decide how to run a plan: the canonical project (after optional-dependency pruning and use_api_socket resolution), the observed daemon state, and the reconciled Plan itself — without creating, starting, or otherwise touching a single container, network, or volume. create() is now just preparePlan + the existing emitRunningEvents + the existing executePlan, in the same order as before.

This is a pure extraction: every step, every error short-circuit, and every side effect (the orphan-containers warning, the unmanaged-network/volume warnings) keeps its exact original position and order. No caller-visible behavior changes — verified against the existing unit suite and the create/up/down e2e scenarios (TestLocalComposeUp, TestCreateIdempotentDefaultPlatform, TestCreateIdempotentSharedImageMixedPlatforms, TestCreateRefusesJob, TestCreateLocalMultiPlatformImage, TestUpWithAllResources).

This closes the last item of epic #14081's Lot 1 — inert code, no consumer yet: nothing calls preparePlan directly outside create() until Lot 2 wires the interactive-up phase split in.

Part of #14081.

🤖 Generated with Claude Code

[docker-agent]

Assessment: 🟢 APPROVE

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/compose/create.go	66.66%	5 Missing ⚠️

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[Copilot]

Copilot review overview

🟢 Approval recommended

The extraction preserves the existing operation order and error handling without introducing behavioral changes.

Review effort: Balanced
Findings: None

What changed in this PR

Extracts create planning into preparePlan, enabling future lifecycle phase separation without changing current behavior.

Changes:

• Adds preparePlan to resolve resources, observe daemon state, and reconcile a plan.
• Refactors create to prepare, emit running events, then execute.

File	Description
pkg/​compose/​create.go	Separates plan preparation from execution.

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[docker-agent]

Assessment: 🟢 APPROVE

[Copilot]

Copilot review overview

🟢 Approval recommended

The extraction preserves operation order, error propagation, and existing execution behavior.

Review effort: Balanced
Findings: None

[docker-agent]

Assessment: 🟢 APPROVE