feat(flux): verify and document list value support in --set flag

[kriscoleman]

Problem

The --set flag is used to override flux values from the CLI:

ailloy cast github.com/nimble-giant/nimble-mold --set agent.targets="[claude,copilot]"

It's unclear whether this correctly produces a list value that works with the has template function:

{{- if has "claude" .agent.targets -}}

If SetNestedValue in pkg/mold/flux.go stores "[claude,copilot]" as a string rather than parsing it into a []interface{}{"claude", "copilot"}, the has function will fail.

Requested

1. Verify that --set key="[a,b,c]" correctly produces a list value (not a string)
2. If it doesn't work correctly, add parsing for bracketed list syntax in SetNestedValue
3. Document the correct syntax for setting list values via --set in the CLI help and README

Use Case

Multi-agent molds like nimble-mold use agent.targets as a list and check membership with has. Users need to be able to override this from the CLI:

ailloy cast ./my-mold --set agent.targets="[claude,copilot,cursor]"

Acceptance Criteria

• --set key="[a,b,c]" produces a list value usable with has, range, etc.
• CLI help documents the syntax for setting list values
• Existing string --set behavior is preserved (backward compatible)