[Performance] Move command recall off the Autofix prompt critical path #844

Goal

Ensure prompt construction never waits for PowerShell command enumeration while preserving accurate warm-cache near-match suggestions.

Parent tracking issue: #786
Depends on: #839

Scope

• Keep the prompt path to token extraction, cheap existence checks, memory-only lookup, and ranking.
• On a cold or stale cache, omit near matches for the current turn and schedule refresh only after an actual command-not-found event.
• Do not warm command recall from helper startup or tab selection.
• Prefer a process-scoped master cache keyed by execution source, shell, PATH fingerprint, and profile/environment identity.
• Initially support host PowerShell explicitly; leave undefined WSL/other-shell environments unsupported.
• Bound both profile-aware enumeration and -NoProfile fallback.
• Use retryable single-flight cache entries with cleanup and TTL.

Acceptance criteria

• Prompt assembly awaits zero command-enumeration subprocesses.
• Selecting tabs without Autofix starts no PowerShell processes.
• Concurrent misses for one cache key start one enumeration.
• Both enumeration paths terminate within documented deadlines.
• Failures and timeouts remain retryable.
• Warm-cache ranking matches the existing blocking implementation.
• Existing cmdlets, functions, and aliases are not misclassified as missing.

Explicit tradeoff

The first cold Autofix turn may omit best-effort near-match hints; it must still send the prompt immediately.