fix(copilot): parse complete message instead of delta for display

[pocky]

Summary

• Fix duplicated output in Copilot streaming display — the parser was consuming assistant.message_delta events (streaming chunks without a trailing newline), which caused the PrefixedWriter buffer to accumulate repeated content (e.g. storedstored)
• Switch to assistant.message (the complete message event) instead, which Copilot CLI also emits and carries the full content field; this produces a single correct [OUT] line per step
• Flip assistant.message_delta to the ignored-event list since its delta content is now superseded by the complete message

Changes

Infrastructure

• internal/infrastructure/agents/copilot_provider.go: Switch display event parser from assistant.message_delta/deltaContent to assistant.message/content; remove Delta: true flag from the returned event

Tests

• internal/infrastructure/agents/copilot_provider_parse_display_events_test.go: Rename test suite and update all fixtures to use assistant.message events; add case for missing content field; assert Delta is false on complete-message events; move assistant.message_delta to the ignored-types test table; update error-path and benchmark fixtures

Changelog

• CHANGELOG.md: Document bug fix B016 — Copilot streaming display duplication root cause and resolution

Test plan

• Run make test and verify all parseCopilotDisplayEvents tests pass with the updated fixtures
• Execute a Copilot workflow step and confirm output appears once (not duplicated) in the [OUT] display line
• Verify assistant.message_delta events are now silently ignored during stream processing
• Run make lint to confirm no linter violations

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