refactor(acp): keep protocol implementation exports private

[juliusmarminge]

The private ACP package exports individual RPC definitions and implementation helpers that no repository consumer imports.

Keep those definitions local and remove the unused client layer wrapper. The live client factory, child-process layer, agent constructor/layers, and RPC groups remain exported. The agent constructor has a local @public explanation because the repository's Effect service convention requires public construction APIs. Generated upstream protocol definitions are untouched.

Verification on the integrated stack:

• ACP Knip export scan passes.
• All 40 existing ACP tests pass.
• ACP typecheck, targeted lint, and formatting pass.

No tests or runtime protocol behavior changed. This is the first cleanup layer; #10171 enforces exports across all seven internal packages.

Model: gpt-6 astra. Harness: Codex in T3 Code.

[macroscopeapp]

Approvability

Verdict: Approved at 913f038

Macroscope's review found this PR approvable — This is a localized private-API cleanup that hides implementation details without changing ACP protocol behavior or existing in-repository call paths. The only removed helper is an unused client layer wrapper, while the supported factories and RPC groups remain available.

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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	13.6 KiB	+2 B (+0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.0 KiB	7.0 KiB	+1 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.6 KiB	+1 B (+0.0%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.1 KiB	57.1 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	10	10	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.5 KiB	−12 B (−0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.0 KiB	7.0 KiB	+4 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.5 KiB	−16 B (−0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.8 KiB	57.8 KiB	−44 B (−0.1%)	66.4 KiB	✅
Claude	Live turn messages	9	8	−1 (−11.1%)	21	✅

Baseline: 62ed748 · PR result: 913f038 · 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: 113.8 KiB
• Claude decoded thread snapshot: 114.5 KiB

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

Dismissing prior approval to re-evaluate 913f038