feat: add billing: "subscription" config option to suppress inflated costs - #1281

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@rust/crates/ccusage/src/config.rs` around lines 278 - 281, The current config
application sets shared.mode = CostMode::Subscription when shared.billing ==
BillingType::Subscription, but that can be overridden later by CLI `--mode`;
move or reapply this enforcement after CLI flags are parsed (or, alternatively,
when merging CLI flags into config, ignore/override any CLI-provided mode if
shared.billing == BillingType::Subscription). Concretely, ensure the invariant
is enforced post-CLI-parse by checking shared.billing ==
BillingType::Subscription and then setting shared.mode = CostMode::Subscription
(or rejecting a conflicting CLI `--mode`) so billing remains authoritative.

Check if this issue is valid — if so, understand the root cause and fix it. At rust/crates/ccusage/src/adapter/opencode/parser.rs, line 127:

<comment>Subscription mode is recognized for pricing-skips but OpenCode cost calculation still returns stored positive `cost_usd`, so subscription runs can report non-zero costs.</comment>

<file context>
@@ -124,7 +124,7 @@ fn missing_open_code_pricing(
     pricing: Option<&PricingMap>,
 ) -> Option<String> {
-    if mode == CostMode::Display || cost_usd.is_some_and(|cost| cost > 0.0) {
+    if mode.skips_pricing() || cost_usd.is_some_and(|cost| cost > 0.0) {
         return None;
     }
</file context>

Check if this issue is valid — if so, understand the root cause and fix it. At rust/crates/ccusage/src/config.rs, line 279:

<comment>`billing: "subscription"` does not reliably force zero-cost mode because `--mode` parsing can overwrite it after config is applied.</comment>

<file context>
@@ -261,9 +268,17 @@ fn is_report_command(command: &str) -> bool {
         apply_shared_options(shared, SharedOptions::from_map(options));
     }
+    // Billing is a stronger signal than mode: subscription always forces zero costs.
+    if shared.billing == BillingType::Subscription {
+        shared.mode = CostMode::Subscription;
+    }
</file context>

Check if this issue is valid — if so, understand the root cause and fix it. At rust/crates/ccusage/src/adapter/opencode/loader.rs, line 45:

<comment>Subscription mode is not fully enforced for OpenCode: embedded positive `cost` values still pass through as non-zero.</comment>

<file context>
@@ -42,7 +42,7 @@ pub(crate) fn load_entries_from_directory(
     shared: &SharedArgs,
 ) -> Result<Vec<LoadedEntry>> {
-    let pricing = if shared.mode == CostMode::Display {
+    let pricing = if shared.mode.skips_pricing() {
         None
     } else {
</file context>

Check if this issue is valid — if so, understand the root cause and fix it. At rust/crates/ccusage/src/adapter/codebuff/loader.rs, line 47:

<comment>Codebuff `Display` mode is not honored: token pricing is still calculated unless mode is exactly `Subscription`.</comment>

<file context>
@@ -40,10 +41,19 @@ fn load_entries_inner(shared: &SharedArgs, pricing: &PricingMap) -> Result<Vec<L
 ) -> LoadedEntry {
-    let cost = calculate_codebuff_cost(&entry, pricing);
-    let missing_pricing_model = missing_codebuff_pricing(&entry, pricing);
+    let cost = if mode == CostMode::Subscription {
+        0.0
+    } else {
</file context>