revert(executor): move OpenRouter engine off main until TASK-313+ are ready

[alekspetrov]

Summary

Revert the TASK-312 engine + factory wire-up from main. The engine is preserved on `feat/openrouter-engine` and will return when its follow-up tasks land.

Why

The OpenRouter engine landed on main via PRs #3205/#3208 (GH-3202) but is dormant code at runtime:

• `DefaultBackendConfig()` still returns `Type: "claude-code"`
• Nothing on main sets `Type: "openrouter"` (verified via `git grep`)
• The engine is only reachable through the factory if a user manually edits their config

Per the TASK-312 design doc, the engine is meant to land with its follow-ups, not standalone:

• TASK-313 — port `bash_validation` (6 submodules) into `engineExecBash`
• TASK-314 — port `file_ops` guards into engine file tools
• TASK-315 — delete the 5 other backends, collapse factory
• TASK-316 — bench wiring
• TASK-317 — delete Python `engine.py`

Shipping the engine alone leaves ~1000 LOC of code that can be activated by a single config-file line but lacks the hardening (bash_validation, file_ops guards) that production use needs. One-line config drift could activate an unfinished engine.

What this PR does

• Reverts commit `5e9298ca` ("feat(executor): wire OpenRouter engine into backend factory (TASK-312: Wire engine.go (OpenRouter backend) into backend_factory.go #3202)")
• Net effect: removes `internal/executor/engine.go`, `internal/executor/engine_test.go`, `.agent/tasks/TASK-312-engine-go-openrouter.md`, and the `case BackendTypeOpenRouter` from `backend_factory.go`
• The duplicate commit `ad44ba24` (identical content from the second PR merge) revert was a no-op and isn't included

Where the engine lives now

Branch `feat/openrouter-engine` (pushed). When TASK-313+ are ready, the engine returns to main via that branch (rebase or PR).

Test plan

• `go build ./...` clean after revert
• `go test ./internal/executor/...` clean — no test references `BackendTypeOpenRouter` from outside the engine's own test file
• Reviewer confirms the engine is dormant on main today (no config sets `openrouter`)

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