chore(adapter/copilot): remove OpenTelemetry data source after #1174

What do you want to change?

Now that #1174 makes session-state the default Copilot data source, the OTel file-export path adds maintenance burden without giving users data they can't already get from session-state. Propose to delete the OTel parser, its env vars, and the OTel↔session-state coexistence dance in a follow-up PR.

Why?

After #1174, session-state is strictly more useful than OTel for ccusage's aggregation purposes:

Data	session-state	OTel	Used by ccusage
Per-model token totals	✅	✅	yes
`totalNanoAiu` (true AI-credit bill, post-2026-06-01)	✅	❌	yes — drives `auto` / `credits` / `display`
`requests.cost` (pre-cutover premium-requests)	✅	❌	yes — drives `auto` for pre-cutover sessions
Per-API-call trace/span IDs	❌	✅	no — ccusage aggregates by day/month/session
Per-call duration / latency	❌	✅	no
Distributed-tracing parent/child links	❌	✅	no
Available without setup	✅ (written on every install)	❌ (env var + exporter setup required)	n/a

OTel's remaining advantages — per-call granularity and distributed-tracing context — are real, but ccusage surfaces none of them. Anyone who actually wants the per-call view should run a proper OTel pipeline (Jaeger, Tempo, an OTel collector), not file-export + ccusage.

What we pay to keep OTel after #1174:

~500 LoC of parser code with chat-span / inference-log / agent-turn / agent-summary-span dispatch and priority-based dedup logic.
COPILOT_OTEL_DEDUP=loose|strict|off env var, invented specifically because we cannot verify whether OTel's session_id (resolved from gen_ai.conversation.id / copilot_chat.session_id / session.id / github.copilot.interaction_id / gen_ai.response.id in priority order) matches the session-state directory UUID. Fragile and unverified by definition.
COPILOT_PREFER_OTEL=1 inversion that lets users opt in to the worse data source.
Three extra env vars in the user-facing surface plus a coexistence section in the Copilot guide.

Removing OTel closes both #1174 and #956 cleanly and eliminates the unverified ID-namespace assumption.

How? (optional)

Delete the OTel functions in rust/crates/ccusage/src/adapter/copilot/parser.rs (parse_otel_file, CopilotUsageSource, CandidateSets, to_candidate, the is_*_record family, and the trace-context helpers).
Remove the Otel variant from CopilotSource in paths.rs and drop COPILOT_OTEL_FILE_EXPORTER_PATH reading.
Remove apply_coexistence_rule from loader.rs; the loader iterates session-state files directly.
Remove COPILOT_OTEL_DEDUP_ENV and COPILOT_PREFER_OTEL_ENV constants and their tests.
Update docs/guide/copilot/index.md, docs/guide/environment-variables.md, and docs/guide/configuration.md to drop OTel mentions.

Breaking-change scope: users with OTel-only historical data and no session-state would lose the ability to analyze that data via ccusage. Realistic impact is small — session-state predates the AI-credits cutover and ships on every Copilot CLI install — but worth calling out in the changelog.

Refs #1174, #956

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