talm template -I silently strips operator comments above modeline

[lexfrei]

Problem

talm template -I -f nodes/node.yaml rewrites the node file in place with the rendered chart output. Any operator-added prose comments above the modeline (project documentation, ticket links, "this node was reset on X for Y" notes) are silently lost — -I is full overwrite, not merge.

Documented behaviour by design (-I flag = rewrite), but the loss is silent: no warning, no count of stripped lines, no preserved-comment block. Operators trip on it regularly.

Reproduction

cd $PROJECT
cat > nodes/node-test.yaml << 'EOF'
# This node was reset 2026-05-12 after ticket OPS-1234 — see internal docs.
# DO NOT edit values directly; modify values.yaml and re-template.
# talm: nodes=["1.2.3.4"], endpoints=["1.2.3.4"], templates=["templates/controlplane.yaml"]
machine:
  certSANs:
    - 1.2.3.4
EOF
talm template -I -f nodes/node-test.yaml
head -3 nodes/node-test.yaml  # operator comments gone, only modeline remains

Verified on the dev17 project: prose comments at the head of nodes/node0.yaml (documenting the OCI / cozystack VIP arrangement) were stripped on the first -I run. Backup restored from git.

Expected — pick one

1. Preserve a top-of-file comment block (# ... lines before the talm modeline), prepending them back after rewrite.
2. Emit a stderr warning with a line count: Warning: removed N lines of operator comments above the modeline. Use --no-strip-comments (TODO) to preserve.
3. Silently fine, document loudly in the talm template -I help and README so operators learn before they lose data.

Option 1 is the most operator-friendly and small in scope (parse the first comment block, re-emit on output before the modeline).

Why this matters

The rewrite path runs unattended in CI / scripted flows; comment loss is invisible until git diff catches it. Operators who don't git commit the change first lose the comments permanently.

Surfaced during the dev17 manual test plan exercise.