Add evolution_plan.md: roadmap for API reliability, request-size guard, and defaults consolidation

[arty-kk]

Motivation

• Capture an audit-derived baseline of architecture, critical flows, pain points, and constraints for the project.
• Define clear reliability and UX goals (fixing API 4xx/5xx semantics, authoritative request-size enforcement, and planner exception handling) and a north-star for defaults consolidation.
• Provide an incremental, testable roadmap and atomic task specs (EVO-001..EVO-007) to reduce drift and surface internal defects early.

Description

• Add evolution_plan.md describing the baseline, north-star outcomes, a phased roadmap (Phase 1..3), and detailed task specifications for seven prioritized work items.
• Enumerate concrete fixes and acceptance criteria for API error semantics, ASGI receive-wrapper body-size guard, resident planner exception narrowing, centralized defaults, CLI UX simplification, shared test helpers, and new contract tests.
• Include validation commands and non-goals to constrain the scope to reliability/UX/testing items and avoid touching compression math or external integrations.

Testing

• No automated tests were executed for this documentation-only change.
• The plan recommends running pytest -q and existing CI checks to validate the future code changes described in the document.

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Codex Task