feat(linter): add file/line to findings; downgrade extends missing-script to warning

Every finding now carries the source file and exact line number of the job
key in its YAML file. Format: [ERROR] job "name" (file.yml:12): message.

Pipeline-level findings (workflow rules, no stages) reference p.SourceFile.
Cross-file include jobs (local, project, component) carry the include source
as their File, set via Pipeline.SetJobOrigin after each ParseBytes call in
the resolver.

Line numbers come from the yaml.Node key node (exact job-name line) in a
new document-level first pass in ParseBytes, replacing the previous
map[string]yaml.Node approach which only gave value-node lines.

Also: jobs that declare extends: but have no script after resolution now
emit WARNING instead of ERROR. The script may come from a base in a remote
include that was not fetched (no token, offline), making the error a false
positive in common project setups.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/linter/dependencies.go

@@ -24,6 +24,8 @@ func checkDependencies(p *model.Pipeline) []Finding {
 				findings = append(findings, Finding{
 					Severity: Error,
 					Job:      name,
+					File:     job.File,
+					Line:     job.Line,
 					Message:  fmt.Sprintf("'dependencies' references unknown job %q", dep),
 				})
 				continue
@@ -34,6 +36,8 @@ func checkDependencies(p *model.Pipeline) []Finding {
 					findings = append(findings, Finding{
 						Severity: Error,
 						Job:      name,
+						File:     job.File,
+						Line:     job.Line,
 						Message:  fmt.Sprintf("'dependencies' job %q must be in an earlier stage (in %q, current job is in %q)", dep, depJob.Stage, job.Stage),
 					})
 				}