fix(template): handle empty or non-mapping products.yml cleanly

[plusky]

What

load_template returned YAML(typ='safe').load(f) verbatim: an empty or
comment-only products.yml loads as None and a top-level sequence loads
as a list. Both crash consumers far from the parse site with
AttributeError on .keys()/.items():

• the REPA shell-completion callback _complete_repa promises that any
  config failure collapses to an empty completion list, but it only
  caught OSError/YAMLError, so the AttributeError escaped into the
  user's shell mid-keystroke;
• KnownProducts._srun aborted 'repose known-products' with a raw
  traceback instead of a clean empty listing.

Fix at the source: load_template now normalizes None to {} so an
empty or comment-only template means 'no products' everywhere, and
raises TemplateError (a ValueError subclass) naming the offending
type for any other non-mapping top level. _dispatch translates
TemplateError into the same one-line error + exit 2 that the other
config-load failures (missing file, permission denied, YAMLError)
already get, so every command fails cleanly instead of with a raw
traceback; subclassing keeps unrelated ValueErrors raised during
command execution (unresolvable REPAs, host repository read
failures) out of that handler. _complete_repa additionally swallows
ValueError so a malformed template still collapses to empty
completions. known-products with an empty template now lists nothing
and exits 0.

Regression tests, all failing on the pre-fix code: the loader and
the completion callback each cover the empty, comment-only and
top-level-sequence templates; known-products covers empty and
comment-only (clean empty listing, exit 0) plus the sequence case
(TemplateError propagates out of run() for _dispatch to translate);
a CLI-level test drives known-products end to end with a list-shaped
products.yml and asserts the one-line error + exit 2 with no
traceback.

Note: this branch and the fix(cli) no-color branch from this batch both touch repose/cli.py and will conflict; either order merges with a trivial rebase, which I'll provide.

Verification

Full gate green (ruff format/check, ty, pytest: 557 passed, coverage 82.53%). Tests at three layers: loader (empty / comment-only / sequence), completion (collapses to empty), known-products (empty / comment-only / sequence), plus a CLI end-to-end test asserting a list-shaped products.yml gets the clean one-liner and exit 2 instead of a traceback. The non-mapping error is a dedicated TemplateError (ValueError subclass) so _dispatch does not mislabel unrelated ValueErrors. Verified to fail pre-fix. Reviewed by a fan-out adversarial code review (two finder lenses per branch, two verifier lenses per finding); all confirmed findings were addressed before opening.

AI-assisted.