feat(mcp): optional per-session working dir for host tools

[localai-bot]

Adds an optional, runtime-only working directory to the host tools so a caller can run bash + filesystem operations rooted at a chosen folder instead of the process cwd. Prerequisite for per-workspace working directories in dante-desktop.

What

• types.Config.WorkingDir string (yaml:"-", runtime-only) — empty means the process cwd (today's behavior).
• bash: bgJobManager carries a dir; launch sets cmd.Dir only when non-empty. New NewShellJobsInDir(dir).
• filesystem: newFileSystem(root) + a resolve helper; every input.Path access is rooted at root when the path is relative. StartFileSystemMCPServer(ctx, transport, root).
• StartTransports threads cfg.WorkingDir into both tools.

Guarantees

• Backward compatible: empty WorkingDir / empty root preserves the exact current process-cwd behavior. No existing caller changes.
• Not a sandbox: only relative paths are rooted; absolute paths and .. still traverse — unchanged from today.

Tests

• bash runs in a configured dir; empty dir uses process cwd.
• filesystem resolves relative→root, leaves absolute unchanged, empty-root verbatim.
• go test ./mcp/... green (73 pass); go build ./... clean.

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