[feature] quest_accept should warn when called before guild_session_start

[kunallanjewar]

Summary

When quest_accept is called without first calling guild_session_start, the call may operate against an unintended (or no) project context. Today the accept succeeds silently and the caller cannot tell whether it landed against the active project. A pre-flight check plus a clear error or hint would prevent invisible cross-project mistakes.

Affected files

• internal/quest/accept.go
• internal/mcp/tools_quest.go (MCP handler entry)

Acceptance

• quest_accept detects when no active project is set and returns a typed error or actionable hint pointing the caller to guild_session_start.
• Existing flows that do call guild_session_start first are unaffected.
• Regression test asserts the warning path.