Emit a valid YAML stream from logs -o yaml

[puncsky]

Summary

render logs -o yaml marshalled each entry with its own yaml.Marshal call and wrote them back to back. Two or more entries therefore produced a single mapping with duplicate keys:

id: a
message: first
id: b
message: second

Strict parsers reject this; permissive parsers silently keep only the last entry, so automation gets an incomplete result while the command exits 0.

nonInteractiveLogs now writes every entry — listed history and tailed events — through one logWriter whose lazily created yaml.Encoder emits a ----separated multi-document stream. Zero entries still produce no output, each Encode flushes its document so --tail keeps streaming, and JSON and text output are unchanged.

Motivation

Exporting logs as YAML for scripting returned unparseable output whenever the query matched more than one line.

How to verify

• go test ./cmd -run TestLogs — new cmd/logs_output_test.go uses the fake Render API server:
  • a two-entry list decodes strictly (KnownFields) as two documents, in order, with matching ids, messages and timestamps;
  • an empty result stays empty;
  • a tailed stream also decodes as two documents;
  • JSON and text output are byte-for-byte unchanged.
• The two YAML stream tests fail on main.
• Full go test ./... passes except e2e/TestE2E, which fails identically on main without credentials.

Release notes

`render logs -o yaml` now prints one YAML document per log entry, so multi-entry output parses correctly.