feat(globaltool): add in-tool build runner (PR-A of bootstrapper deprecation)

[ChrisonSimtian]

Re-opened from #200, which closed because GitHub Actions wouldn't trigger workflows on its branch (`feat/global-tool-in-tool-runner`) despite multiple pushes, an empty-commit nudge, and a close/reopen cycle. Same code, fresh branch.

Summary

First of three PRs that deprecate the per-repo `build.cmd`/`build.sh`/`build.ps1` bootstrappers in favour of `Fallout.GlobalTool` (`fallout`) as the canonical entry point for running builds. This one is purely additive — no behaviour change for any repo with existing bootstrappers committed.

Three-PR sequence:

• PR-A (this one) — Add in-tool build runner. `fallout ` runs `dotnet build` + `dotnet run --no-build` directly instead of spawning `./build.sh`.
• PR-B — Switch this repo to thin shims, delete `build.cmd`, update the `[GitHubActions]` generator + workflows, update `:setup` scaffolder.
• PR-C — Docs cleanup (already tracked under Docs CLI cleanup: replace stale nuke references with fallout across docs/ #198).

What's in this PR

New files:

• `src/Fallout.GlobalTool/BuildProjectResolver.cs` — resolves the build project (`.csproj`) from optional `BuildProjectFile` field in `.fallout/parameters.json`, falling back to convention `build/_build.csproj`. Six unit tests cover convention default, explicit override, missing files (both cases), and empty override values.
• `src/Fallout.GlobalTool/Program.Run.cs` — new in-tool runner. Resolves `dotnet` (PATH first; shim-provisioned `.fallout/temp/dotnet-{unix,win}` as fallback), seeds the same env vars the shim sets, then runs `dotnet build` + `dotnet run --project --no-build -- ` as subprocesses with inherited stdio (colours/progress pass through).
• `tests/Fallout.GlobalTool.Tests/BuildProjectResolverTests.cs` — 6 new tests, all green.

Modified:

• `src/Fallout.GlobalTool/Program.cs` — `Handle`'s no-`:` fallback now calls `Run(args, rootDirectory, BuildProjectResolver.Resolve(rootDirectory))` instead of `Build(buildScript, ...)`. The `buildScript == null` branch of the "Could not find" prompt is removed — we can run without a shim now — but the `rootDirectory == null` branch stays for empty repos.

What this PR does NOT do (so PR-B has something to do)

• Doesn't delete `Build()` — still called from `Program.Complete.cs` as a bootstrap path for the first-time completion request. Removed in PR-B alongside the shim deletion.
• Doesn't touch the shim scripts (`build.sh`/`build.ps1`/`build.cmd`) — they continue to work directly via their own `dotnet build` + `dotnet run --project` invocations, independent of the global tool.
• Doesn't touch the `[GitHubActions]` generator — it still emits `run: ./{BuildCmdPath} {targets}`, which still works because the shims are still in place. The generator and the four `.github/workflows/*.yml` files get the three-step `setup-dotnet` + `tool restore` + `dotnet fallout` shape in PR-B.
• Doesn't add `BuildProjectFile` to `build.schema.json` — the schema is regenerated by `HandleShellCompletionAttribute` on every build run, so manual edits don't stick. The schema-generation update belongs to PR-B. The resolver works without schema support because `parameters.json` accepts arbitrary fields (JSON Schema default allows additional properties).

Backwards compatibility

Scenario	Behaviour
Existing repo, old fat bootstrapper, run `./build.sh Compile`	Unchanged — bootstrapper does its own `dotnet build` + `dotnet run --project`, doesn't touch the global tool.
Existing repo, old fat bootstrapper, run `fallout Compile`	Now uses the new in-tool runner. Same end result (`dotnet build` + `dotnet run --no-build`), one fewer subprocess hop. The shim's env-var exports are bypassed; the new runner sets them itself to compensate.
Existing repo, no `_build.csproj` (somehow)	Resolver throws with a helpful message pointing at `fallout :setup` or `BuildProjectFile` in `parameters.json`.
Empty directory, no `.fallout/`	Prompt for `:setup` as before.

Verification

• `dotnet build src/Fallout.GlobalTool/Fallout.GlobalTool.csproj` — green (0 errors; only pre-existing warnings unrelated to this PR).
• `dotnet test tests/Fallout.GlobalTool.Tests/` — 17/17 passed (11 existing + 6 new).
• Dogfood: `dotnet run --project src/Fallout.GlobalTool -- --help` — exercises full new path; outputs build help identical to `./build.sh --help`.

Test plan

• CI green on `ubuntu-latest` (the only PR gate).
• `Fallout.GlobalTool.Tests` green on all three release runners post-merge — though note: `windows-latest` is currently broken on `main` due to a separate pre-existing issue with `Nuke.Common.Shim.Tests` having no discoverable test methods (filing a separate PR for that).

Refs

• This PR: PR-A of the bootstrapper-deprecation plan.
• Next: PR-B will flip this repo to thin shims + `actions/setup-dotnet` + `dotnet fallout`, and update the `[GitHubActions]` generator + `:setup` scaffolder.
• Companion docs sweep: tracked in Docs CLI cleanup: replace stale nuke references with fallout across docs/ #198.

[ChrisonSimtian]

Tracked under umbrella issue #203 — bootstrapper deprecation plan. Pick-up checklist for tomorrow lives there.