feat(agent-acp): support remote HTTP and SSE MCP servers

[huntharo]

Summary

I made @pwrdrvr/agent-acp authoritative for ACP v1 MCP server configuration
across stdio, streamable HTTP, and HTTP+SSE transports.

I added:

• a backward-compatible public MCP config union plus exact pinned-protocol wire types
• env/header normalization from records or { name, value }[] collections
• explicit HTTP/SSE capability normalization and gating helpers
• per-thread MCP forwarding for session/new, session/load, and the existing reopenThread path
• pooled-client isolation for MCP payloads and approval-policy server names
• lifecycle error redaction for HTTP URLs, header values, and stdio env values
• a minor Changeset and consumer migration documentation

The pinned @zed-industries/agent-client-protocol@0.4.5 exposes session/new
and session/load, but no separate session/resume, so I did not invent a
hot-reconfiguration or resume method outside the ACP spec.

Why

PwrAgent currently carries its own ACP stdio/HTTP/SSE union and capability gate,
while PwrSnap is constrained by the stdio-only public type in
@pwrdrvr/agent-acp@0.12.2. This change lets both hosts use the shared package
for per-thread remote HTTP MCP injection without another custom ACP client or an
HTTP-to-stdio shim.

Migration notes

PwrAgent

I expect PwrAgent to replace its local MCP config union with
AcpMcpServerConfig. Existing HTTP/SSE
headers: Array<{ name, value }> values are accepted unchanged. It can keep
gating through the normalized agentCapabilities.mcp.http / .sse data or use
acpRuntimeSupportsHttpMcp, acpRuntimeSupportsSseMcp,
client.supportsHttpMcp(), and client.supportsSseMcp().

PwrSnap

I expect PwrSnap to pass its selected MCP list per thread through
startThreadNative, reopenThread, or loadThreadNative. A shared
AcpAgentClientPool remains safe: session payloads and approval context contain
only that thread's configured servers.

Stdio callers need no migration. Their existing
{ name, command, args?, env?: Record<string, string> } inputs remain valid.

Verification

I verified the complete repository CI sequence locally:

• pnpm lint:licenses
• pnpm lint:deps
• pnpm lint:boundaries
• pnpm build
• pnpm typecheck
• pnpm test — 296 passing, 1 pre-existing skip

The focused agent-acp suite has 146 passing tests, including exact validation
against the pinned ACP mcpServerSchema, HTTP/SSE lifecycle payloads,
capability gating, pooled two-thread isolation, in-flight session/load history
replay and rollback, and credential redaction/non-logging.