#155 feat(codemode): replace Yaegi with Expr

Replace the Go/Yaegi CodeMode runtime with Expr on feat/expr-codemode.

• execute CodeMode as Expr expressions
• expose only the codemode runtime API
• preserve exact tool whitelist validation and orchestration
• support sequential tool chaining with let
• collect streaming results through CallToolStream
• remove Yaegi-specific tests and docs
• add Expr runtime tests

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Summary by cubic

Replaces the CodeMode Go/Yaegi interpreter with expr-lang/expr, so generated workflows are now Expr expressions instead of Go-like snippets. Code that relied on __out, imports, type assertions, or manual stream loops must be rewritten; tool whitelist validation and orchestration caching stay unchanged.

• Adds github.com/expr-lang/expr v1.17.8, drops github.com/traefik/yaegi, and makes the orchestrator fully Expr-native.
• Extracts the plan JSON from model output even when wrapped in prose or markdown, then validates snippets with Expr before execution.
• The planner prompt now signals the Expr contract explicitly so generated programs are unambiguously non-Go.
• Generated programs see only the codemode API object, with no Go imports or Yaegi stdlib.
• codemode.CallToolStream collects streaming chunks into an array instead of a manual iterator.
• Removes Yaegi benchmarks and preprocessing tests; replaces them with Expr runtime coverage.

Migration

• Rewrite generated code as let-bound sequential expressions where the last expression is the result.
• Read tool-result fields with codemode.Get instead of Go type assertions.
• Replace return/__out patterns with the expression-oriented result model.