perf: cache builtin alias map and DFS cycle-check to fix CompileMCPWorkflow regression

[Copilot]

BenchmarkCompileMCPWorkflow regressed +14% (432µs vs 379µs). Two hot paths were doing redundant work on every compilation: BuiltinModelAliases() deep-copied the 52-entry builtin map on every call, and detectCircularModelAliases ran a full DFS over all 52 builtin keys even though builtins are immutable and cycle-free.

model_aliases.go

• Added builtinOnlyAliasMap() backed by sync.Once returning a cached shared reference to the merged builtin map
• MergeImportedModelAliases(nil, nil) — the common case for MCP workflows with no imports — now returns the cached reference instead of deep-copying

model_alias_validation.go

• detectCircularModelAliases uses a new isBuiltinOnlyAliasMap helper (key-set equality, not just length) to detect when the input is the builtin-only map
• When builtin-only, DFS runs once via sync.Once and the result is cached for all subsequent calls
• loadBuiltinModelAliases() errors are now logged with a fallback to full DFS instead of being silently discarded
• Cached DFS call passes empty markdownPath to avoid embedding a stale caller path in a shared error

Result: ~291–306µs vs reported 432µs (~30% faster); 80 fewer allocs/op, ~10KB less memory per compilation.

[github-actions]

🤖 PR Triage

Field	Value
Category	refactor (perf)
Risk	🟡 Medium
Score	58/100 (Impact 28 + Urgency 15 + Quality 15)
Action	🔄 batch_review
CI	⚪ Unknown (draft, no checks yet)
Batch	perf-regressions (with #44994, #44995)

Draft PR — part of perf regression batch. Review together with #44994 and #44995.

Generated by 🔧 PR Triage Agent · 180.8 AIC · ⌖ 7.62 AIC · ⊞ 5.6K · ◷

[github-actions]

🤖 PR Triage

Field	Value
Category	refactor / perf
Risk	🟡 medium
Score	45/100 (impact:22 + urgency:13 + quality:10)
Action	📦 batch_review
Batch	pr-batch:perf-regressions (with #44994, #44995)

Rationale: Draft. Caches builtin alias map and DFS cycle-check to fix CompileMCPWorkflow +14% regression. Small change. Batch with other perf PRs.

Triage run §29183606049

Generated by 🔧 PR Triage Agent · 171.6 AIC · ⌖ 5.63 AIC · ⊞ 5.6K · ◷