rendertree: replace or deprecate the delimiter-based --custom mini-language

[apstndb]

Problem

The --custom flag uses an ad-hoc mini-language with two layers of delimiter-based parsing:

• stringList.Set splits the flag value on ,
• customListToTableRenderDef then splits each item on :

Relevant code:

• cmd/rendertree/impl/impl.go:218-225
• cmd/rendertree/impl/impl.go:551-594

This means some otherwise valid Go template expressions cannot be represented on the command line, especially when a column name or template needs a literal , or :.

Why this matters

--custom is documented in the README as a supported way to define custom columns, but the current format is much more fragile than --custom-file.

Examples that are awkward or impossible today:

• string literals containing : or ,
• templates that want to emit those characters literally
• future extensions that need additional structured fields without making the grammar even more ambiguous

Expected behavior

The CLI should provide one of these:

1. A structured repeatable flag format that does not depend on raw , and : splitting.
2. Escaping/quoting rules for the current mini-language.
3. A clear deprecation path for --custom in favor of --custom-file.

Suggested direction

I would prefer deprecating --custom and steering users toward --custom-file, unless there is a strong need to keep the inline syntax.

If --custom stays, it should become a repeatable structured flag rather than a hand-rolled CSV-like grammar.