Refine CLI defaults and agent auth handling

[jmgilman]

Motivation

• Centralize fallback behavior for storage paths and base image selection so CLI commands behave consistently when no config is loaded.
• Allow the --agent flag to be specified without an explicit value and validate agent names independently of a loaded config.
• Reduce duplication and complexity in agent credential retrieval by consolidating provider metadata and error handling.

Description

• Exported the default base image as DefaultBaseImage and updated Viper defaults and tests to use it instead of an unexported symbol.
• Added defaultDataDir and resolveBaseImage helpers and wired them into initManager, logs and recreate flows so defaults come from a single place.
• Simplified credential injection by introducing an agentAuthSpecs map and unified injectAuthToken logic, and enabled --agent to accept no option value via NoOptDefVal with sentinel handling.
• Replaced config-dependent agent validation with package-level helpers (config.IsValidAgent / config.ValidAgentNames) and updated call sites accordingly.

Testing

• Formatted modified files with gofmt to ensure style consistency and applied the changes to source and tests.
• Updated unit tests to reference DefaultBaseImage where applicable but no automated go test run in this rollout.
• No automated test failures were observed because the test suite was not executed as part of this change.

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Codex Task

[cloudflare-workers-and-pages]

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	headjack	bb7bed0	Commit Preview URL Branch Preview URL	Jan 02 2026, 10:49 PM