fix: clean pause on transient Ollama capacity/overload without burning escalation

[tzone85]

Problem

Running multiple concurrent builds against one Ollama instance, the server returns transient overload/resource conditions: HTTP 429/503, `server busy`, `no slots available`, `maximum pending requests exceeded`, `model is loading`, `out of memory`, `context deadline exceeded`, `connection refused`. These were never classified, so the pipeline treated them as story-quality failures and only paused the requirement after burning the whole escalation chain (reset → manager diagnosis → tech-lead re-plan), with misleading messages ("agent produced no code changes", "re-plan failed").

Fix

• `internal/llm/errors.go` — `IsCapacityError(err)` classifies 429/503/529 (typed `*APIError`) and stringified Ollama HTTP-client errors carrying a capacity signature. `ContainsCapacitySignature(string)` is the shared vocabulary. Capacity is distinct from `IsFatalAPIError` (401/403/billing — permanent) and from a 404 model-not-found (operator config error). Signatures are the exact strings `internal/llm/ollama.go` and the Ollama server emit.
• `internal/engine/capacity_pause.go` — `Monitor.pauseIfCapacity(storyID, stage, err)` pauses the requirement without consuming an escalation attempt or advancing the tier. Wired into: reviewer, merge/conflict-resolution, manager diagnosis, tech-lead re-plan. `agentCompletionHasCapacityError(storyID)` scans the latest `STORY_COMPLETED` payload's `error` field so a native (Gemma) agent that hit an overload (empty diff) pauses cleanly instead of escalating as "no code changes". The pause reason states the cause is transient — resume after the server recovers.

Tests

• `internal/llm/capacity_test.go` — classifier coverage (signatures + non-signatures, not-fatal invariant).
• `internal/llm/ollama_test.go` — a live 503/429 from the Ollama server is classified as capacity, not fatal.
• `internal/engine/capacity_pause_test.go` — `pauseIfCapacity` (pauses without escalation / returns false otherwise), `agentCompletionHasCapacityError`, and post-execution review + empty-diff paths assert REQ_PAUSED with zero STORY_REVIEW_FAILED.

TDD throughout: tests written first, watched fail, then implemented.

Green gate

```
go build -o /tmp/nxd ./cmd/nxd && go vet ./... && go test ./... -count=1 # exit 0, all 30 packages ok
golangci-lint run ./... # 0 issues
```