Adds an instances option to run configurations (dev environments, tasks, services) that restricts a run to specific existing fleet instances.

Syntax

Long forms:

instances:
  - fleet: my-fleet
    instance: 3
  - name: my-fleet-1
  - hostname: worker-1

Short form for matching by instance name:

instances:
  - my-fleet-1

The fleet form also supports <project name>/<fleet name> for fleets from another project.

Behavior

• instances has allow-list semantics: a run is placed only on a matching existing instance.
• When instances is set, dstack never provisions new instances to satisfy the run.
• If no matching instance is available, the run fails with a no-capacity error; retry can be used to wait for a selected busy instance to free up.
• A run is rejected up front if it specifies fewer instances than required by its node count.
• New-capacity backend offers are skipped when instances is set because they cannot satisfy the selector.

Implementation

• Adds strict selector models to ProfileParams: name, hostname, and fleet + instance, while preserving the string shorthand as an instance-name selector.
• Reuses the existing fleet/instance offer selection path and filters loaded instances by the selected instance selectors.
• Supports both backend fleets and SSH fleets.
• Supports qualified fleet references without broadening query complexity for the common unqualified case.
• Keeps the change backward compatible by omitting unset instances for older client/server compatibility paths.
• No DB schema change; the field is stored in the existing run/profile JSON.

Docs

Updated the shared fleet-management snippet and protips guide. The docs promote the explicit syntax first and keep the short instance-name syntax in a collapsible section.

Testing

• uv run ruff check .
• uv run pyright -p .
• uv run pytest — 2607 passed, 1055 skipped
• Local end-to-end testing with dstack server:
  • Backend AWS fleet: baseline fleets plus all four instances syntaxes completed successfully.
  • SSH fleet: created an EC2 instance from the dstack AWS AMI, configured it as an SSH fleet, and verified all four instances syntaxes completed successfully.
  • Negative case: nonexistent instance selector failed with “Failed to use specified instances” and did not provision another backend instance.

AI Assistance

This PR includes AI-assisted changes. The original PR noted Claude Code assistance; follow-up schema, implementation review, tests, docs, and E2E verification were assisted by Codex.