Add opt-in stateless HTTP transport mode

[tmunzer-AIDE]

Summary

Adds an opt-in stateless HTTP transport (MISTMCP_STATELESS env / --stateless flag). With --transport http, the server is served via mcp_server.http_app(stateless_http=True) under uvicorn — a fresh transport per request, no server session id — so an already-connected MCP client survives a server restart without reconnecting. Default off ⇒ existing behavior is byte-for-byte unchanged.

Because the per-request transport starts already-initialized, on_initialize can't be relied on to set per-session state, so the port also:

• Resolves write-tool visibility at build time in create_mcp_server() (_configure_write_visibility): hides write + write_delete by default and re-shows write only in the DANGER ZONE. Behavior-neutral in stateful mode (the elicitation middleware still re-resolves per session); required for stateless so the DELETE tool can't leak or auto-execute.
• Sets the DANGER-ZONE elicitation bypass request-scoped in a new on_call_tool (gated on config.stateless), so auto-accept works without an initialize and doesn't leak into the session store.
• Fails closed deterministically: config_elicitation_handler raises ElicitationUnavailableError (converted to a clean ToolError by the existing tool wrappers) when elicitation is needed in stateless HTTP, instead of relying on ctx.elicit()'s undefined stateless behavior.

Safety gate: startup is refused (non-zero exit) for exactly stateless AND http AND enable_write_tools AND NOT disable_elicitation — the one combo that would need in-band elicitation. Also adds a MISTMCP_DISABLE_ELICITATION env var (symmetric with MISTMCP_ENABLE_WRITE_TOOLS) so env-only deployments can reach the writes-in-stateless (DANGER ZONE) config.

Accepted trade-offs (documented in the README): no server→client push (GET route dropped), and destructive upgrade/utility actions fail closed in read-only stateless.

Design spec and implementation plan are included under docs/superpowers/.

Test Plan

• uv run python -m pytest — 97 passed, coverage 61% (gate ≥30%)
• uv run ruff check src tests / ruff format --check — clean on all changed files
• New tests cover: the validation refusal matrix; env/CLI/start() threading + main() non-zero exit; build-time visibility matrix + idempotency (via real list_tools()); on_call_tool request-scoped state; the fail-closed guard; and the _run_stateless_http launch (asserts http_app(stateless_http=True) with no event_store + exact uvicorn args)
• Default-off behavior verified byte-for-byte unchanged (non-stateless HTTP launch path identical; new branches no-op when stateless=False)
• Manual smoke: uv run mistmcp --transport http --stateless (logs the stateless INFO line; restart-survival); --stateless --enable-write-tools without --disable-elicitation exits non-zero with the refusal message

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