[V2:08] Resolve Workspace Execution Policy

[ritorhymes]

Part of V2: Multi-repo local build system.

Summary

This PR adds the execution-policy layer that centralizes how runtime commands resolve the active repository, source materialization, sibling sources, build roots, and base URLs.

Default build, serve, and check runs use the tracked active local working tree with editable local workspace sources. --clean switches only the active repository to clean materialization. --remote-siblings switches sibling repositories to their configured remote sources while retaining the active local source. --build-root selects the prepared output root, and --base-url provides a build/serve rendering override.

It does not add the prepared runtime pipeline, static preview serving, or targeted rendering. Review should focus on default-local policy, clean active-source handling, remote-sibling selection, build-root/base-URL resolution, and command/flag applicability.

• Add execution.rs with ResolvedExecution, ExecutionSettings, and runtime source-selection policy.
• Resolve the active repository through its required Build.toml configuration.
• Make default build, serve, and check runs use the tracked active local source plus editable local workspace sources.
• Add --clean to materialize a clean active source without changing local-theme or sibling-source policy.
• Add --remote-siblings to use configured remote sibling sources without forcing clean active-repository materialization.
• Add centralized --build-root and --base-url resolution for runtime callers.
• Route runtime commands through the resolved execution policy.
• Reject execution-only overrides on non-runtime commands such as print, init, and doctor.
• Add CLI and execution-policy tests.

Review Notes

This PR is the policy layer between workspace setup and the runtime pipeline. Later runtime PRs consume ResolvedExecution instead of independently deciding where sources, output, and rendering settings come from.

The default runtime contract is local-first: the active proposal repository comes from the tracked local working tree, and workspace-local sources remain editable for normal development. A valid active-repository Build.toml is required.

--clean changes active-source materialization only. It validates and materializes a clean active source while preserving the local development workspace model, including the editable local theme.

--remote-siblings changes sibling-source selection only. It uses the active repository locally while obtaining declared sibling repositories from their configured remote URLs. It does not imply --clean.

Build-root resolution is centralized so later build, serve, and preview callers use the same prepared output location. An explicit --build-root overrides the workspace default.

Base-URL resolution is centralized for rendering callers. An explicit --base-url wins over the configured default where the command supports it.

Targeted selection remains intentionally out of scope. V2:15 adds targeted build rendering, and V2:16 extends that behavior to serve.

Verification

• Review src/execution.rs for active-repository configuration, local-first source selection, clean active-source materialization, remote-sibling selection, build-root resolution, base-URL resolution, and command applicability.
• Review src/cli.rs for runtime-only execution flags and rejected non-runtime command forms.
• Review src/main.rs for runtime dispatch through resolve_execution.
• Review src/changed.rs for changed-file listing using the resolved source policy.
• Review CLI and execution-policy tests for default local behavior, --clean, --remote-siblings, build-root/base-URL handling, invalid command forms, and missing or invalid active-repository configuration.