feat(cache): add cache token audit logging behind OPENCODE_EXPERIMENTAL_CACHE_AUDIT

[martinffx]

Issue for this PR

Closes #5416, #5224
Related: #14065, #5422, #14203, #14743

Part of the stacked PR decomposition of the prompt caching fix (1 of 4). Adds cache token audit logging as observability foundation, the metrics logged here are what we use to verify cache hit improvements in subsequent PRs.

Type of change

• New feature

What does this PR do?

Adds cache token audit logging behind OPENCODE_EXPERIMENTAL_CACHE_AUDIT=1, providing per-LLM-call cache hit/miss visibility. This PR is purely observability — it does not change any caching behavior.

Changes:

1. packages/core/src/flag/flag.ts — adds OPENCODE_EXPERIMENTAL_CACHE_AUDIT flag.

2. packages/opencode/src/session/session.ts — adds rawInputTokens to the usage return value (sum of input + cache read + cache write), needed by both the log and sidebar display.

3. packages/opencode/src/session/processor.ts — after each LLM call's getUsage(), if the flag is set, logs a [CACHE] line with total input tokens, cache read/write/new breakdown, hit percentage, and output tokens.

4. packages/opencode/src/cli/cmd/tui/feature-plugins/sidebar/context.tsx — when the flag is set, displays a "Cache Audit" box in the TUI sidebar showing the same token breakdown and hit rate in real time.

Why this PR first: The subsequent PRs in the stack improve cache hit rates. This PR provides the measurement tooling to verify those improvements work.

How did you verify your code works?

Set OPENCODE_EXPERIMENTAL_CACHE_AUDIT=1 bun dev, send a prompt, and confirm:

• A [CACHE] log line appears per LLM call in the log output
• The sidebar shows the "Cache Audit" box with token breakdown

Tests: bun test packages/opencode/test/session/session.test.ts passes.

Screenshots

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

Co-authored-by: Bhagirathsinh Vaghela bhagirathsinh.vaghela.001@gmail.com

[github-actions]

Hey! Your PR title Feat/cache audit logging doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

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[github-actions]

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