feat(codex): implement F103 NDJSON output parity for Codex provider

[pocky]

Summary

• Closes the F103 output parity gap for Codex: state.Output and ConversationResult.Output now contain clean aggregated assistant text extracted from NDJSON streams instead of raw JSON envelopes, regardless of output_format value — matching the behavior Claude, Gemini, and OpenCode already had since 0.7.1.
• Introduces presence-aware overwrite logic: An empty assistant_message event (hadText=true, text="") is correctly distinguished from non-NDJSON plain-text fallback (hadText=false), preventing inflated token estimates and stale turn Content when the model responds with empty output.
• Populates state.Response for Codex via tryParseJSONResponse on the extracted assistant text, giving Codex the same automatic structured-output heuristic already present in other providers and enabling {{.states.step.Response.field}} in downstream workflow steps.
• Docs updated to reflect that state.Output stores extracted text (not raw NDJSON) for all CLI providers and that state.Response is now populated heuristically across all six providers.

Changes

Infrastructure — Codex Provider

• internal/infrastructure/agents/codex_provider.go: Wire extractCodexTextContent hook into cliProviderHooks; add extractCodexAssistantText (presence-aware NDJSON aggregator returning (string, bool)); add extractCodexTextContent adapter for the base hook signature; override Execute and ExecuteConversation with F103 presence-aware overwrite of Output, Response, token counts, and trailing turn Content; simplify NewCodexProvider to delegate to NewCodexProviderWithOptions; document NUL-byte preservation divergence from Claude provider.
• internal/infrastructure/agents/helpers.go: Migrate findFirstNDJSONEvent and findLastNDJSONEvent from strings.Split to strings.SplitSeq (allocation-free iterator).

Tests

• internal/infrastructure/agents/codex_provider_unit_test.go: Add ~450 lines of new unit tests covering all F103 paths: presence-aware overwrite, output_format-invariant extraction, Response population, empty assistant_message, multi-event aggregation, NUL byte preservation, non-JSON plain text, and token recount; remove superseded timestamp/provider-name duplicate tests and stub-era expectedTokens hardcoding.
• internal/infrastructure/agents/codex_provider_delegation_test.go: Remove all stub-era conditional if err != nil { return } guards; harden assertions with require.NoError; add TestCodexProvider_ExecuteConversation_OutputExtraction, HookWired, PlainTextFallback, and NDJSONOutput delegation tests; assert errors.Is(err, context.Canceled) on cancellation paths; remove no-op TestCodexProvider_Validate_BinaryNotFound.
• internal/infrastructure/agents/codex_provider_test_helpers_test.go: New file — shared ndjsonLine helper that JSON-escapes arbitrary text into a valid Codex item.completed NDJSON envelope.

Documentation

• docs/reference/interpolation.md: Expand Response section to cover agent steps with examples showing {{.states.step.Response.field}}; clarify JSON vs Response semantics and when each is populated.
• docs/user-guide/agent-steps.md: Update state.Output description to reflect extracted-text semantics for CLI providers; add F103 Output Parity note to the display-filtering table; correct "raw NDJSON" references in silent-mode note.
• README.md: Update output-formatting feature bullet to mention automatic state.Response population.
• CHANGELOG.md: Add [Unreleased] entry for F103 with full parity description.

ZPM Knowledge Base

• .zpm/kb/pr_feature_f103_codex_provider_jsonl_output_parity/journal.wal: PR tracking journal.
• .zpm/kb/pr_feature_f103_codex_provider_jsonl_output_parity/knowledge.pl: PR Prolog schema for blocking-issue tracking.

Test plan

• Run make test — all unit and integration tests pass with zero failures and no data races (make test-race)
• Run make lint — zero violations; verify nolint:errcheck directives carry explanatory comments
• Execute a Codex agent step in a workflow that pipes {{.states.codex_step.Output}} into a downstream step and confirm clean text (not raw NDJSON) is interpolated
• Execute a Codex agent step with a prompt that returns JSON and verify {{.states.codex_step.Response.field}} resolves correctly without requiring output_format: json

Closes #363

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