fix(unified): count OpenCode Claude Agent turns once - #2005

Implements the accepted fix for #2004: unified reports count OpenCode claude-agent turns once, on the Claude side.

What changed:

• rust/adapters/opencode/src/parser.rs: OpenCodeV2Message now parses content[].native.sessionID, gated on the claude-agent provider; session_message_value_to_entry returns the linked ids alongside the entry so the loader filters without a second parse.
• rust/adapters/opencode/src/loader.rs: new load_entries_for_unified skips v2 rows whose native session has a Claude transcript on disk. Skipped sessions still count as covered, within the same database and across data directories, so the cumulative session_v2 aggregate cannot bring the usage back. Without a transcript the rows stay. Focused ccusage opencode keeps every row via the unchanged load_entries.
• rust/adapters/claude/src/paths.rs: new public transcript_session_ids reporting modern <project>/<session>.jsonl stems, failing open (legacy agent-*.jsonl copies, nested paths, non-UTF-8 paths, and unreadable config dirs are left out so usage is kept, never dropped on a guess).
• rust/crates/ccusage-adapter-all/src/loader.rs: unified OpenCode spec loads through load_entries_for_unified, honoring the same claude_project filter as the Claude side.
• Docs: short notes in the OpenCode guide and adapter README.

Scope notes: the legacy message table and storage/message/*.json fallback are untouched (they predate the v2-only claude-agent provider); the reported opus alias overpricing and the unrelated session-window/Windows issues from the thread are out of scope.

Tests:

• New: parser native-link tests (claude-agent link, other-provider ignore, missing link), loader tests (skip with transcript, keep without, aggregate suppression incl. cross-directory archive root, focused load unaffected), Claude transcript_session_ids tests, and an end-to-end unified daily test (Claude 100 in + OpenCode 50 in counted, 100-token duplicate and its cost skipped).
• cargo fmt --check clean, cargo clippy --all-targets clean (CI gates on -D warnings), full cargo test --workspace: 63 suites, all passing.
• Network-free pricing was unavailable (no nix here), so tests ran with CCUSAGE_PRICING_JSON_PATH pointed at the flake-lock-pinned LiteLLM snapshot downloaded once to /tmp.

Changes are left uncommitted in the working tree as instructed.

Closes #2004

Summary by cubic

Fixes unified reports double-counting OpenCode claude-agent turns. The provider runs Claude Code under the hood, so each turn was stored twice: in OpenCode's database and in Claude Code's own transcript. Unified reports now skip the OpenCode copy when the transcript is on disk, counting the turn once, on the Claude side — which also carries the resolved model id that OpenCode's opus alias lacks.

• Skipped sessions still count as covered within the same database and across data directories, so the cumulative session_v2 aggregate cannot bring the usage back.
• ccusage opencode keeps every row, and when a transcript is gone the OpenCode rows are counted instead, so no usage disappears.
• Skips apply only to v2 claude-agent rows with a content[].native.sessionID; legacy message-table rows and other providers are unaffected.

^{Written for commit ccfaff6. Summary will update on new commits.}