Say what a build was made from, and hand an image compiling Go only that (hq ADR 0267, issue 363)

A merge to the controller's repository moved the route proxy and the build
seat's holder whatever it changed, because nothing said which files their
builds read. A build of a trunk commit now says its build source per
repository: a Go program's import closure, an archive's directory, an
image's recipe and the package it names in the new 'compiles' field. That
image is built from its build source alone, so a recipe reading past it
fails by name, and its fingerprint is over what it was handed.

internal/link/build.go

@@ -224,6 +224,12 @@ type BuildResult struct {
 	// manifest the mesh keeps says nothing about it (novox/hq 04-ISSUES/131).
 	Read []ReadRepository `json:"read,omitempty"`
 
+	// Sources are what the build was made from, as files, per repository (novox/hq ADR 0267): the
+	// entries the planner maps the next merge's changed files onto. Said only for a build of a commit on
+	// the trunk, and only for a repository whose every artifact's build source the builder knows; a
+	// repository it says nothing of is read whole, as before. Empty from a builder that predates it.
+	Sources []BuildSource `json:"sources,omitempty"`
+
 	// Trunk is the repository's default branch at the build, and OnTrunk whether the commit built is on it
 	// (novox/hq ADR 0238): **only a commit on the trunk is published** — the controller refuses to register
 	// a build of one off it. Empty Trunk is a build seat that could not say, or predates the rule.
@@ -263,6 +269,16 @@ type ReadRepository struct {
 	Ref        string `json:"ref,omitempty"`
 }
 
+// BuildSource is the build source a build read in one repository (novox/hq ADR 0267): Repository and Ref
+// a context's, as its manifest names it, and empty for the module's own; Paths the entries, relative to
+// that repository's root — `dir/` a Go package's directory but its tests, `dir/**` everything below a
+// directory, `**` the whole tree, anything else one file (builder.SourceHolds).
+type BuildSource struct {
+	Repository string   `json:"repository,omitempty"`
+	Ref        string   `json:"ref,omitempty"`
+	Paths      []string `json:"paths"`
+}
+
 // MadeArtifact is one thing a build produced, as a person would want it reported.
 type MadeArtifact struct {
 	Name      string `json:"name"`