Expose the build model as data: fallout describe --json and plan --json

[ChrisonSimtian]

Problem

To learn what a Fallout build can do, a tool has to read Build.cs, follow its partial classes, resolve the attributes, and re-run the dependency resolution in its head. --help renders that same information as human text, and there is no machine-readable form of it. There is also no way to ask what a target would run without running it.

Outcome

Two read-only commands that print the build model as JSON and never execute a target.

• fallout describe --json — every target, the dependency edges (DependsOn, Before, After, Triggers), every parameter with its CLR type, default value and required flag, tool requirements, and descriptions.
• fallout plan <target> --json — the ordered list of targets that would run, with skip reasons. This is the existing plan resolution with the executor not attached.

Acceptance criteria

• fallout describe --json emits a schema-versioned document covering targets, dependency edges, parameters, and tool requirements
• fallout plan <target> --json emits the resolved plan and invokes no target
• Both commands leave the working tree untouched and run no external tool
• Both exit non-zero with a machine-readable error when the build project fails to load
• --help is rendered from the same model, so the human and machine views cannot drift

Notes

src/Fallout.Build/Execution/ExecutionPlanner.cs already computes the plan, then discards it into the console. #391 (the Serilog and NDJSON output work) names describe --json as a motivation but does not track it.

[ChrisonSimtian]

Part of #648 (the AI-native build interface epic), milestone AI-native build interface.