feat(executor): "Opus plans, Sonnet executes"

[alekspetrov]

Parent: GH-2432

full implementation - Implement the entire epic as a single consolidated task to avoid merge conflicts. The bulk of changes (Phases 1-4, 6) all touch internal/executor/ (config.go, backend.go, backend_claudecode.go, runner.go, epic.go, hooks.go), and the retry-label work in Phase 5 (internal/github/labels.go, internal/poller/poller.go, internal/autopilot/auto_merger.go) is tightly coupled to the executor changes via shared constants and label lifecycle. Splitting would cause cascade conflicts in runner.go (6 call sites) and config.go (shared struct extensions). Deliverables: (a) extend ClaudeCodeConfig with AllowedTools/MCPConfigPath and add PlanningConfig with claude-opus-4-7 default; (b) extend ExecuteOptions and append --allowedTools/--mcp-config in args builder; (c) populate options at all 6 backend.Execute sites via a buildExecutorOptions helper; (d) modify PlanEpic to pass --model claude-opus-4-7, restrict tools to Read,Grep,Glob, and set ANTHROPIC_MODEL env per-invocation; (e) replace in-memory retryReadyCount with GitHub labels (pilot-retry-1/2/exhausted) and clean them up on merge in auto_merger.go; (f) flip RunTestsOnStop default to false; (g) add tests for planning-model routing, AllowedTools/MCPConfigPath wiring, and retry-label state; (h) verify go build ./... && go test ./... pass.

[alekspetrov]

❌ Pilot execution failed:

execution failed: subtask 1/5 failed: unknown: exit status 1

[alekspetrov]

❌ Pilot execution failed:

execution failed: subtask 1/5 failed: unknown: exit status 1

[alekspetrov]

Duplicate of parent #2432. Sub-issue scope ('implement entire epic as single consolidated task') is identical to the parent, which is already pilot-in-progress. Closing to avoid double-execution.