Refactor oa2csv flags to Config struct

[Copilot]

Consolidates 9 scattered flag variables in cmd/oa2csv/main.go into a single Config struct for better organization and maintainability.

Changes

• Added Config struct with fields for all CLI parameters (Env, Token, Instrument, Granularity, Price, FromStr, ToStr, OutPath, CompleteOnly)
• Migrated flag declarations from flag.String() return values to flag.StringVar() populating struct fields
• Updated references throughout main() from pointer dereferencing (*token) to struct field access (cfg.Token)

Before:

var (
    env = flag.String("env", "practice", "...")
    token = flag.String("token", "", "...")
    // ... 7 more flags
)
flag.Parse()
if *token == "" {
    *token = os.Getenv("OANDA_TOKEN")
}

After:

cfg := &Config{}
flag.StringVar(&cfg.Env, "env", "practice", "...")
flag.StringVar(&cfg.Token, "token", "", "...")
// ... 7 more flags
flag.Parse()
if cfg.Token == "" {
    cfg.Token = os.Getenv("OANDA_TOKEN")
}

CLI interface remains unchanged.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>All flags need to use Config files.</issue_title>
<issue_description>Turn flag variables into config structures.</issue_description>

<agent_instructions>Make all the flag variables in cmd/oa2csv/main.go info a single struct `type Config struct {...}. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes All flags need to use Config files. #13

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