feat(stack)!: require an explicit stack ID instead of guessing one

[patrickhilker]

TL;DR: Stack-aware commands no longer guess a stack. The stack ID must be passed via --stack-id/-s (or argument) or set in the context. The default-stack special handling is removed. This is a breaking change and will result in a major release.

Motivation

Projects no longer get a default stack, and existing default stacks can now be deleted. The CLI used to rely on the default stack in two ways:

• container run, volume create and volume delete fell back to the project ID as stack ID when no stack was given. Without a default stack this fails with an opaque API error.
• stack delete only emptied a project's default stack instead of deleting it.

The root cause is a design flaw in the CLI, not in the API: the API modelled stacks from the very beginning, so that the stack feature could later be introduced properly without breaking the API. The CLI, however, wrongly relied on every project always having a default stack (with the project ID as its stack ID). This change removes that assumption.

Additionally, all stack-aware commands silently picked a project's stack if it was the only one. Implicitly choosing a stack (e.g. for volume delete) is risky, and scripts relying on it break as soon as a second stack is created.

Changes

• Remove optionalStackFlags / withStackIdOrDefault; container run, volume create and volume delete now use the regular stackFlags / withStackId
• Remove the auto-selection of a project's only stack from withStackId
• stack delete always calls deleteStack; --with-volumes is deprecated and has no effect
• volume create checks that the stack belongs to the given project (the project ID was only used for the default-stack fallback before)

Breaking changes

• container run, volume create, volume delete, stack deploy, stack ps, stack delete and stack template install fail with No stack ID given. Please specify one with --stack-id or set a default stack with 'mw context set --stack-id <stack-id>' if no stack ID is given via flag, argument or context — even if the project has only a single stack.
• stack delete removes a project's default stack instead of emptying it.

Testing

yarn lint, yarn compile and yarn test pass. There are no existing tests covering stack resolution or the affected commands; the change was not tested against the live API. The docs in docs/*.md will be regenerated by the README workflow.

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