feat(codemode): replace Yaegi with Expr

[Raezil]

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

---

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.

^{Written for commit 73be66c. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found and verified against the latest diff

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="src/plugins/codemode/codemode.go">

<violation number="1" location="src/plugins/codemode/codemode.go:89">
P1: When `stream.Next()` fails mid-stream, `CallToolStream` currently returns partial items with a nil error, so transport/runtime failures look like success. Handle EOF as completion and return non-EOF errors instead.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

P1: When stream.Next() fails mid-stream, CallToolStream currently returns partial items with a nil error, so transport/runtime failures look like success. Handle EOF as completion and return non-EOF errors instead.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/plugins/codemode/codemode.go, line 89:

<comment>When `stream.Next()` fails mid-stream, `CallToolStream` currently returns partial items with a nil error, so transport/runtime failures look like success. Handle EOF as completion and return non-EOF errors instead.</comment>

<file context>
@@ -1,567 +1,160 @@
+	stream, err := api.client.CallToolStream(api.ctx, name, args)
+	if err != nil { return nil, fmt.Errorf("CallToolStream failed: %w", err) }
+	items := make([]any, 0)
+	for { item, err := stream.Next(); if err != nil { return items, nil }; items = append(items, item) }
+}
+
</file context>