fix: restrict set_skywalking_url tool to stdio transport only

[wu-sheng]

Summary

• Restrict set_skywalking_url session tool to stdio transport only, reverting the session-in-all-transports behavior from PR feat: add session-based SkyWalking URL and basic auth support #29
• SSE and HTTP transports no longer inject a shared Session{} into context, avoiding the issue where one client could override another's URL/credentials
• SSE/HTTP transports continue to use per-request SW-URL headers and --sw-url CLI flag for URL configuration
• Update docs (README, CLAUDE.md) to clarify transport-specific URL resolution

Context

PR #29 review feedback (from both Copilot and @Fine0830) identified that SSE and HTTP transports shared a single Session{} across all clients, which is a correctness and security issue. Since stdio has a single client with well-defined session semantics, set_skywalking_url is now stdio-only.

Test plan

• Verify swmcp stdio registers set_skywalking_url tool and session works as before
• Verify swmcp sse and swmcp streamable do NOT expose set_skywalking_url tool
• Verify SSE/HTTP transports still resolve URL from SW-URL header > --sw-url flag > default
• CI passes (license + lint + docker build)

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