[codex] Unify Persona Runtime prompt context pipeline

[murphys7017]

Summary

• replace the Prompt Selector/Profile path with one canonical ContextPack and deterministic Router, Persona, and Core projections
• add versioned context building, explicit plugin-context collection, conflict detection, and a provider-neutral PromptTreeBuilder
• preserve official conversation history and group context while isolating persona/effect semantics from Core execution
• keep structured user content parts intact and persist scaffold-free conversation messages
• update Yakumo architecture documentation and the execution flow

Why

Prompt construction had accumulated separate local paths for Router, Persona Expression, Core execution, plugin contexts, and provider rendering. Those paths could silently overwrite context, leak target-specific extensions, or lose structured content. This change makes data collection canonical and moves target differences into explicit projection rules.

Validation

• 463 prompt, interaction, group-context, and main-agent unit tests passed
• 138 postprocess, memory-runtime, and tool-loop tests passed
• Ruff checks passed for all affected implementation and test modules
• Python compile checks, YAML parsing, document encoding, and git diff --check passed

Notes

This branch also contains the ten previously committed Persona Runtime changes that were ahead of origin/master, so the PR represents the complete current local architecture state.

[sourcery-ai]

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