fix: MCP safe reads do not retry transport failures or preserve diagnostics

[viowai]

What Happened

@everme/agent-sdk@0.3.4 documents one retry for safe transport failures, but the retry branch is unreachable for native fetch failures:

1. execOnce() catches the native failure and wraps it in EvermeError.
2. execWithRetry() immediately rethrows every EvermeError.
3. A synthetic local socket-reset probe therefore observes one request, not two.

There is a second semantic gap: /mem/search and /mem/context are safe reads implemented as POST, so method-only retry gating cannot cover them. When the error reaches @everme/memory-mcp, the adapter returns only error: <message> and drops correlation/classification data that an MCP host could use for health decisions.

This is separate from EverMind-AI/EverOS#338, which tracks ingestion task metadata and exact visibility verification.

Steps To Reproduce

1. Use @everme/agent-sdk@0.3.4 or current main at 0e6fd0ec90a2cf8a90946f434504b6813826eb1a.
2. Point createClient() at a local HTTP server that resets the first GET connection and succeeds on the second.
3. Call client.request("GET", "/healthz").
4. Observe that the server receives one request and the caller receives EvermeError immediately.

The same dead path applies to DNS/connection errors. POST /mem/search and POST /mem/context are never eligible under the current method-only check.

Expected Behavior

• Explicit semantic reads (GET/HEAD, plus POST /mem/search and /mem/context) should make at most one bounded retry for transient transport failures, HTTP 429, and HTTP 5xx.
• All attempts and Retry-After delay should share the original timeout budget.
• Non-idempotent memory writes must remain single-attempt, even if a caller mislabels one as a read.
• Exhausted errors should expose redacted fields such as classification, cause code, HTTP status, request id, attempts, retryability, and elapsed time.
• MCP tool failures should preserve those fields in structuredContent.error while retaining the existing isError text response.
• Diagnostics must not include queries, request bodies, URLs, tokens, or native fetch cause text.

Environment

• Package: @everme/agent-sdk@0.3.4, @everme/memory-mcp@0.3.4
• Runtime: Node.js 18+
• Reproduction: synthetic localhost transport failure; no production data involved

Proposed Fix

• Add explicit safe_read / non_idempotent_write request semantics.
• Allow POST safe-read opt-in only for a hard allowlist of /mem/search and /mem/context.
• Retry safe reads once within a shared deadline; do not start a second full timeout.
• Preserve structured, sanitized EvermeError metadata through the MCP adapter.
• Add regression coverage for socket reset, DNS failure, timeout budget, 429, 5xx, 401/auth classification, and write single-attempt behavior.

Logs

No production logs are attached. The reproduction uses placeholder credentials and a local synthetic endpoint only.