feat(sandbox): offer to link directory after interactive picker

[codyde]

What

When railway sandbox commands fall through to the interactive workspace → project → environment picker (i.e. no --project/--environment flags and no existing link), the picker now ends by offering to save the selection as the directory link:

? Link this directory to my-project (production)? (Y/n)

• Yes (default): persists via the same link_project path railway link uses, prints Linked to my-project (production). Run railway unlink to undo. Future commands — sandbox and otherwise — skip the prompts.
• No / Esc: proceeds without linking; the sandbox operation continues unaffected.

Why

The sandbox commands already honor an existing link (precedence: flags → linked project/environment → picker), but in an unlinked directory the full three-level picker re-runs on every create/list/template invocation. The user just deliberately picked a project and environment — offering to remember it costs one Enter keypress.

Design notes

• Explicit consent, not silent linking. Only railway link and railway init write the link today; this keeps that precedent. The link is durable state that steers railway up/variables, so it shouldn't be mutated as a side effect.
• Picker path only. The prompt never appears when the project came from a flag or an existing link — passing explicit flags signals the user doesn't want directory state touched.
• resolve_project_and_env/resolve_target/prompt_workspace_project_env take &mut Configs now; all callers already held one.

Testing

• cargo check + cargo test (288 passed)
• Not yet manually exercised — suggested verification: unlinked dir → sandbox create → picker → accept link → second sandbox create should go straight through; railway unlink restores the picker

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