[Phase 4.4] Context chaining between phases

[fernandoxavier02]

Task

Executor resolves {phase_N.result} references in prompts before invoking skills.
Each phase output is captured, summarized, and available as context for subsequent phases.

TDD

• Test reference resolution with mock phase outputs
• Test missing reference handling
• Test nested references

Acceptance Criteria

• {phase_1.result}, {phase_2.result} etc. resolved correctly
• Missing references produce clear error, not silent empty string
• Context accumulates across phases without exceeding budget

Design Ref

Phase 4, task 4.4