fix(stats): count forked sessions' copied messages once (backport #306)

[slkiser]

Summary

Backport of the #306 fix to the 4.x (OpenCode 1) line.

• OpenCode 1's Session.fork clones each parent message into the fork with a new id but the same times, model, tokens and cost. The all-history, time-window and multi-session readers counted each copy again, so forking a $10 session without using the fork showed $20 in token/cost totals.
• dropForkCopies in src/lib/opencode-storage.ts counts each finished fingerprint (created/completed time, provider, model, tokens, cost) as many times as the session that has it most often, and drops the extra copies. It also runs on the no-json_extract fallback of the completed-message reader. Unfinished rows are never merged.
• A single session's own read is unchanged.
• Known limits: two unrelated sessions with exactly the same created/completed millisecond, model, tokens and cost are counted once; a copied reply with usage but no completion time (only after a crash mid-reply) still counts twice.

Linked Issue

Fixes #306

OpenCode Validation

• Current production released OpenCode version tested: not tested live on OpenCode 1; covered by unit tests. The same fix was tested live on OpenCode 2.0.20 (v5 PR).
• Why this version is relevant to the fix: the reporter uses OpenCode 1.18.34, whose fork clones message info unchanged (verified in its Session.fork source).

Before-and-after evidence

Not applicable for screenshots on OpenCode 1 (not run live). The new tests fail on the old 4.x code (5 fork-copy tests) and pass with this change.

Surface checks:

• /tokens_* output: covered by unit tests only.
• TUI sidebar, toast, compact line: not applicable; they show provider quota, not local token history.

Quality Checklist

• I ran pnpm verify
• This change is focused and avoids unrelated behavior changes
• I updated or added tests when behavior changed
• I updated docs when user-facing workflow, command, or config behavior changed (no docs describe how totals are counted)
• For provider changes, I followed Provider Changes, or this does not apply