perf(shared): avoid duplicate command discovery probes

[rynfar]

Repeated PATH entries made editor and provider discovery probe the same files repeatedly. Skip exact duplicate directories and retain a single trace for the enclosing lookup. Case-distinct directories, lowercase executable extensions, and the existing cache policy remain supported.

Adopts upstream #9618, c3caceade14ccda0686db0031aca79d7d38a71d0.

Validation: 58 focused shared-shell/external-launcher tests pass; one platform-specific test skips. Shared typecheck, targeted lint, and diff checks pass. Native Windows timing was not measured.

Model: GPT-6 Astra. Harness: Codex.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.8 KiB	+173 B (+1.2%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	−2 B (−0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.7 KiB	+175 B (+2.6%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.5 KiB	57.4 KiB	+910 B (+1.6%)	66.4 KiB	✅
Codex	Live turn messages	8	10	+2 (+25.0%)	21	✅
Claude	Total thread wire	13.7 KiB	13.8 KiB	+133 B (+1.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	−7 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.7 KiB	+140 B (+2.1%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.3 KiB	58.2 KiB	+884 B (+1.5%)	66.4 KiB	✅
Claude	Live turn messages	8	9	+1 (+12.5%)	21	✅

Baseline: 6466561 · PR result: 82812bd · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 111.2 KiB
• Claude decoded thread snapshot: 111.9 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.